Braking control device for vehicle and braking control method for vehicle

ABSTRACT

A vehicle braking control device includes first and second deceleration calculation units, an assist control unit, and a termination determination unit. The first deceleration calculation unit calculates a first estimated vehicle body deceleration using a wheel speed sensor detection signal. The second deceleration calculation unit calculates a second estimated vehicle body deceleration using a vehicle body acceleration sensor detection signal. The assist control unit initiates assist control, which assists increasing a braking force when the first estimated vehicle body deceleration exceeds a first deceleration determination value and the second estimated vehicle body deceleration exceeds a second deceleration determination value. The termination determination unit determines whether or not a termination condition of the assist control is satisfied based on at least one of the first and the second estimated vehicle body decelerations. The assist control unit terminates the assist control when it is determined that the termination condition is satisfied.

TECHNICAL FIELD

The present invention relates to a braking control device for a vehicleand a braking control method for a vehicle that performs assist controlthat assists the increasing of a braking force applied to wheels basedon an operation of a brake pedal by a driver of a vehicle.

BACKGROUND ART

As braking control devices for such a vehicle, in the prior art, patentdocuments 1 and 2 describe examples of braking control devices. In suchbraking control devices, the starting timing for the assist control isset without using a sensor that directly detects the depression forceapplied to a brake pedal by a driver. An example of such a sensor is apressure sensor that detects a master cylinder pressure.

More specifically, in the above braking control device, an estimatedvehicle body speed of the vehicle is calculated based on a detectionsignal from a wheel speed sensor arranged on the vehicle, and anestimated vehicle body deceleration of the vehicle is calculated basedon the estimated vehicle body speed. Further, the braking control devicecalculates a slip rate of a wheel. In a state in which the driveroperates the brake pedal, if the calculated estimated vehicle bodydeceleration is greater than or equal to a preset first emergencybraking determination value and a calculated slip rate of a wheelexceeds a preset second emergency braking determination value, it isdetermined that the current operation of the brake pedal is an emergencybraking operation. This starts an assist control. The assist control isalso referred to as “brake assist control (BA control)”.

Instead of using the slip rate of a wheel, the vehicle body deceleration(also referred to as “G sensor value”) may be calculated based on adetection signal from a vehicle body acceleration sensor arranged in thevehicle, and the G sensor value may be used to determine whether thecurrent brake pedal operation is an emergency braking operation.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    2002-193084-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2002-370634

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The assist control is executed when determined that emergency control isnecessary. Hence, when the driver's operation amount of the brake pedalbecomes small during the assist control, termination of the assistcontrol is preferred.

To terminate the assist control without using a sensor that directlydetects the depression force applied by the driver to the brake pedal, abrake switch that detects the ON/OFF of the brake pedal is used, forexample. In this method, however, when the operation amount of the brakepedal is decreased to reduce the deceleration of the vehicle, theoperation amount does not become small enough to detect that the brakepedal is OFF. Thus, the assist control cannot be terminated. As aresult, a large braking force remains applied to the wheels.

It is an object of the present invention to provide a braking controldevice for a vehicle and a braking control method for a vehicle capableof terminating, at an appropriate timing, assist control that assiststhe increasing of the braking force applied to the wheels based on abraking operation performed by a driver.

To achieve the above object, one aspect of the present inventionprovides a braking control device for a vehicle including a firstdeceleration calculation unit, a second deceleration calculation unit,an assist control unit, and a termination determination unit. The firstdeceleration calculation unit calculates a first estimated vehicle bodydeceleration by using a detection signal of a wheel speed sensorarranged on the vehicle. The second deceleration calculation unitcalculates a second estimated vehicle body deceleration by using adetection signal of a vehicle body acceleration sensor arranged on thevehicle. The assist control unit initiates an assist control, whichassists increasing of a braking force applied to a wheel of the vehicleif a brake pedal of the vehicle is operated, when the first estimatedvehicle body deceleration exceeds a first deceleration determinationvalue and the second estimated vehicle body deceleration exceeds asecond deceleration determination value during operation of the brakepedal of the vehicle. The termination determination unit determineswhether or not a termination condition of the assist control issatisfied based on at least one of the first estimated vehicle bodydeceleration and the second estimated vehicle body deceleration duringexecution of the assist control. The assist control unit terminates theassist control when the termination determination unit determines thatthe termination condition is satisfied.

The above structure determines whether or not a termination condition ofthe assist control is satisfied based on at least one of the firstestimated vehicle body deceleration and the second estimated vehiclebody deceleration during execution of the assist control. When thetermination condition is satisfied, the assist control is terminated.Accordingly, the assist control can be terminated at a proper timing.

Preferably, the assist control includes an increasing control, whichincreases the braking force applied to the wheel, and a holding control,which holds the braking force applied to the wheel after the increasingcontrol is executed. The termination determination unit acquires adetermination value by adding an increase component of the brakingforce, which is based on the execution of the increasing control, to abraking force reference value set as a reference for determining, whenthe assist control has not yet been executed, whether or not anoperation amount of the brake pedal has decreased. The terminationdetermination unit determines, during execution of the holding control,that the termination condition of the assist control has been satisfiedwhen the second estimated vehicle body deceleration calculated by thesecond deceleration calculation unit is less than the determinationvalue.

When the driver's operation amount of the brake pedal changes, thesecond estimated vehicle body deceleration changes in accordance withthe change amount. Thus, the second estimated vehicle body decelerationmay be referred to as a value that indicates the operation amount of thebrake pedal. In the assist control, when the second estimated vehiclebody deceleration becomes less than a braking force reference value, itis preferred that the assist control be terminated. When the driver isoperating the brake pedal in a state in which the assist control is notexecuted, the braking force reference value is set based on thedeceleration generated by the vehicle when the driver's operation amountof the brake pedal becomes small, that is, when the driver decreases thedepression of the brake pedal. However, when the decelerationcorresponding to the increase amount of the braking force applied to thewheel by the assist control, that is, the deceleration based on thedriver's operation of the brake pedal exceeds the braking forcereference value, the assist control cannot be terminated regardless ofhow small the driver's operation amount of the brake pedal is.

Thus, in the present invention, the determination value is obtained byadding an increase component of the braking force, which is based on theexecution of the increasing control, to the braking force referencevalue. When the second estimated vehicle body deceleration exceeds thesecond deceleration determination value, the increasing control and theholding control are sequentially executed. During the holding control,when the second estimated vehicle body deceleration that once exceedsthe second deceleration determination value becomes less than thedetermination value, the driver's operation amount of the brake pedal isdetermined as having been decreased, and the assist control isterminated. Accordingly, the assist control can be terminated at atiming at which it is determined that the driver intends to decrease thedeceleration of the vehicle.

Preferably, an ABS control unit that performs anti-lock braking control,which restricts locking of the wheel, is further included. Thetermination determination unit prohibits, during execution of theholding control, the termination condition of the assist control frombeing satisfied when the ABS control unit is executing the anti-lockbraking control.

When the driver operates the brake pedal at a timing at which the roadsurface along which the vehicle is traveling is switched from a high proad to a low p road, the anti-lock braking control may be initiated.This is because the slip rate of the wheel increases when the roadsurface switches to the low p road. When the anti-lock braking controlis initiated, the braking force applied to the wheel decreases. As thebraking force applied to the wheel decreases, the first estimatedvehicle body deceleration and the second estimated vehicle bodydecoration decrease. In this case, the decrease in each estimatedvehicle body deceleration is caused by the initiation of the anti-lockbraking control, and the probability is high that the drive does notintend to decrease the braking force. Further, after the anti-lockbraking control is initiated, the road surface may be switched againfrom a low p road to a high p road. In such a case, if the driver'soperation amount of the brake pedal remains large, the assist controlshould be continued. Therefore, in the present embodiment, duringexecution of the holding control, the termination condition of theassist control is prohibited from being satisfied when the ABS controlunit is executing the anti-lock braking control. This prevents theassist control from being terminated in an unintended manner.

Preferably, the assist control includes an increasing control, whichincreases the braking force applied to the wheel, and a holding control,which holds the braking force applied to the wheel after the increasingcontrol is executed. The increasing control is executed during a presetincrease requisition time. An ABS control unit that performs anti-lockbraking control, which restricts locking of the wheel, is furtherincluded. When an elapsed time from when the increasing control isinitiated is shorter than a termination determination time referencevalue, which is set to be shorter than the increase requisition time,the termination determination unit determines that the terminationcondition of the assist control is satisfied upon satisfaction of anyone of the conditions of the ABS control unit starting the anti-lockbraking control, and the first estimated vehicle body decelerationcalculated by the first deceleration calculation unit is greater than orequal to a deceleration that corresponds to a road surface limit.

When the anti-lock braking control is initiated immediately after theincreasing control in the assist control is initiated, there is a highprobability that the anti-lock braking control will be initiated becauseof the driver's operation amount of the brake pedal is large. Further,when the first estimated vehicle body deceleration is greater than orequal to a deceleration that corresponds to a road surface limit, thewheel may slip when the braking force applied to the wheel is furtherincreased. In such a case, the driver's operation of the brake pedalapplies sufficient braking force to the wheel. Thus, the need for theassist control is low. Therefore, in the present embodiment, the assistcontrol is terminated when the elapsed time from when the increasingcontrol is initiated is shorter than the termination determination timereference value or when the first estimated vehicle body deceleration isgreater than or equal to a deceleration that corresponds to a roadsurface limit. Thus, the assist control can be terminated at a propertiming.

A braking control method for a vehicle in a further aspect of thepresent invention includes a first deceleration calculating step, asecond deceleration calculating step, an assisting step, and atermination determination step. The first deceleration calculating stepcalculates a first estimated vehicle body deceleration by using adetection signal of a wheel speed sensor arranged on the vehicle. Thesecond deceleration calculating step calculates a second estimatedvehicle body deceleration of the vehicle by using a detection signal ofa vehicle body acceleration sensor provided on the vehicle. Theassisting step initiates an assist control, which assists increasing ofa braking force applied to a wheel of the vehicle if a brake pedal ofthe vehicle is operated, when the first estimated vehicle bodydeceleration exceeds a first deceleration determination value and thesecond estimated vehicle body deceleration exceeds a second decelerationdetermination value during operation of the brake pedal of the vehicle.The termination determination step determines whether or not atermination condition of the assist control is satisfied based on atleast one of the first estimated vehicle body deceleration and thesecond estimated vehicle body deceleration. The assisting stepterminates the assist control when determined in the terminationdetermination step that the termination condition is satisfied.

The above configuration obtains the same operations and advantages asthe above-described braking control device of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a vehicle including a brakingcontrol device according to one embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating the structure of a brakingdevice;

FIG. 3 is a timing chart illustrating a state in which assist control isexecuted when the vehicle is decelerating;

FIG. 4 is a timing chart illustrating a case in which the vehicle istraveling on a sloped road directed uphill;

FIG. 5 is a timing chart taken when the gradient of the road on whichthe vehicle is traveling changes to an uphill side;

FIG. 6 is a map for setting a downshift determination value;

FIG. 7 is a map for setting a gradient change reference value;

FIG. 8 is a flowchart illustrating a braking control processing routine;

FIG. 9 is a flowchart illustrating an information acquisition processingroutine;

FIG. 10 is a flowchart illustrating a BA initiation determinationprocessing routine (first half);

FIG. 11 is a flowchart illustrating the BA initiation determinationprocessing routine (middle stage);

FIG. 12 is a flowchart illustrating the BA initiation determinationprocessing routine (latter half);

FIG. 13 is a flowchart illustrating a BA processing routine;

FIG. 14 is a flowchart illustrating a BA termination determinationprocessing routine;

FIG. 15A is a timing chart showing changes in the vehicle bodydeceleration, and FIG. 15B is a timing chart showing changes in thenoise component;

FIG. 16 is a timing chart illustrating a case in which the vehicle movesover a bump on a road;

FIG. 17 is a timing chart illustrating a case in which the vehicle movesover a bump on a road;

FIG. 18 is a timing chart illustrating a case in which the driverapplies a normal depression force to the brake pedal;

FIG. 19 is a timing chart illustrating a case in which the driverapplies a large depression force to the brake pedal;

FIG. 20 is a flowchart illustrating an initiation time determinationreference value setting processing routine; and

FIG. 21 is a map for estimating the load on a vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

One embodiment of the present invention will now be described withreference to FIGS. 1 to 19. In the following description of the presentspecification, the travelling direction (forward direction) of a vehicleis referred to as the frontward side (front of vehicle).

Referring to FIG. 1, the vehicle of the present embodiment is a frontwheel drive vehicle including a plurality of (four in the presentembodiment) wheels (right front wheel FR, left front wheel FL, rightrear wheel RR and left rear wheel RL), among which the front wheels FRand FL are the drive wheels. The vehicle includes a drive forcegenerator 10, which has an engine 12 serving as one example of a powersource that generates drive force corresponding to the amount of anaccelerator pedal 11 operated by a driver, and a drive forcetransmission device 20, which transmits the drive force generated by thedrive force generator 10 to the front wheels FR and FL. The vehicle isincludes a braking device 30 which applies, to the wheels FR, FL, RR,and RL, a braking force in correspondence with the driver's depressionoperation performed on a brake pedal 31.

The drive force generator 10 includes a fuel injection system (notshown) arranged in the vicinity of an intake port (not shown) of theengine 12 and including an injector for injecting fuel into the engine12. The drive force generator 10 is controlled by an engine ECU 13 (alsoreferred to as “engine electronic control unit”) including a CPU, a ROMand a RAM (none shown). An accelerator operation amount sensor SE1,which is arranged in the vicinity of the accelerator pedal 11 to detectthe amount of the accelerator pedal 11 operated by the driver(accelerator operation amount), is electrically connected to the engineECU 13. The engine ECU 13 calculates the accelerator operation amountbased on a detection signal from the accelerator operation amount sensorSE1 and controls the drive force generator 10 based on the calculatedaccelerator operation amount.

The drive force transmission device 20 includes an automatictransmission 21, which is one example of a transmission, and adifferential gear 22, which appropriately distributes and transmits thedrive force transmitted from an output shaft of the automatictransmission 21 to the front wheels FL and FR. The drive forcetransmission device 20 is controlled by an AT ECU 23 (AT electroniccontrol unit) including a CPU, a ROM and a RAM (all not shown). The ATECU 23 controls the automatic transmission 21 (upshift control anddownshift control) in accordance with the vehicle body speed of thevehicle and the states of the operations performed by the driver onaccelerator pedal 11, the brake pedal 31, and a shifting device (notshown).

As shown in FIGS. 1 and 2, the braking device 30 is includes a liquidpressure generator 32 and a brake actuator 40. The liquid pressuregenerator 32 includes a booster 320, a master cylinder 321, and areservoir 322. The brake actuator 40 includes two liquid pressurecircuits 41 and 42 (shown by double-dashed lines in FIG. 2). The liquidpressure generator 32 includes a brake switch SW1, which detects whetheror not the brake pedal 31 is depressed by a driver. The brake switch SW1sends a detection signal to a brake ECU 60 (also referred to as “brakeelectronic control unit”), which controls the brake actuator 40.

The liquid pressure circuits 41 and 42 are connected to the mastercylinder 321 of the liquid pressure generator 32. A wheel cylinder 55 afor the right front wheel FR and a wheel cylinder 55 d for the left rearwheel RL are connected to the first liquid pressure circuit 41. A wheelcylinder 55 b for the left front wheel FL and a wheel cylinder 55 c forthe right rear wheel RR are connected to the second liquid pressurecircuit 42. The booster 320 and the master cylinder 321 are operatedwhen the driver of the vehicle depresses the brake pedal 31. Thissupplies brake fluid from the master cylinder 321 into the wheelcylinders 55 a to 55 d through the liquid pressure circuits 41 and 42.Then, a braking force corresponding to the wheel cylinder pressure (alsoreferred to as “WC pressure”, hereinafter) in each of the wheelcylinders 55 a to 55 d is applied to the wheels FR, FL, RR, and RL.

Next, the brake actuator 40 will be described with reference to FIG. 2.Since the liquid pressure circuits 41 and 42 have substantially the samestructure, only the first liquid pressure circuit 41 is shown in FIG. 2for the sake of convenience, and the second liquid pressure circuit 42is not illustrated in the drawing.

As shown in FIG. 2, the first liquid pressure circuit 41 includes aconnection passage 43 connected to the master cylinder 321. Theconnection passage 43 includes a normally-open type linear solenoidvalve 44, which is operated to generate a pressure difference between amaster cylinder pressure (also referred to as the “MC pressure”) in themaster cylinder 321 and the WC pressures in the wheel cylinders 55 a and55 d. A left front wheel passage 44 a connected to the wheel cylinder 55a for the left front wheel FR and a right rear wheel passage 44 dconnected to the wheel cylinder 55 d for the right rear wheel RL areformed in the first liquid pressure circuit 41. The passages 44 a and 44d respectively includes pressure increasing valves 45 a and 45 d, whichare operated when restricting increasing of the WC pressures of thewheel cylinders 55 a and 55 d and which are normally-open type solenoidvalves, and pressure decreasing valves 46 a and 46 d, which are operatedwhen decreasing the WC pressures in the wheel cylinders 55 a and 55 dand which are normally-closed type solenoid valves.

A reservoir 47, which temporarily stores the brake fluid that flows outeach of the wheel cylinders 55 a and 55 d through the pressuredecreasing valves 46 a and 46 d, and a pump 49, which is operated by therotation produced by a motor 48, are connected to the first liquidpressure circuit 41. The reservoir 47 is connected to the pump 49through a suction flow passage 50 and connected to the connectionpassage 43 through a master side flow passage 51 at a location closer tothe master cylinder 321 than the linear solenoid valve 44. The pump 49is connected by a supply flow passage 52 to a connecting portion 53between the linear solenoid valve 44 and the pressure increasing valves45 a and 45 d in the first liquid pressure circuit 41. When the motor 48produces rotation, the pump 49 draws in brake fluid from the reservoir47 and the master cylinder 321 through the suction flow passage 50 andthe master side flow passage 51 and discharges the brake fluid into thesupply flow passage 52.

Next, as one example of a depression force estimation device, thebraking control device and the brake ECU 60 will be described withreference to FIGS. 1 and 2.

As shown in FIGS. 1 and 2, the brake switch SW1, wheel speed sensorsSE2, SE3, SE4 and SE5 that detect rotation speeds of the wheels FR, FL,RR, and RL, and a vehicle body acceleration sensor SE6 that detects a Gsensor value, which is the vehicle body deceleration in the front torear direction of the vehicle, are electrically connected to an inputinterface of the brake ECU 60. The motor 48 and the valves 44, 45 a, 45d, 46 a and 46 d forming the brake actuator 40 are electricallyconnected to an output interface of the brake ECU 60.

The vehicle body acceleration sensor SE6 outputs a signal in which the Gsensor value becomes a negative value when the vehicle acceleratesbecause the center of gravity of the vehicle moves rearward and a signalin which the G sensor value becomes a positive value when the vehicledecelerates because the center of gravity of the vehicle moves forward.Thus, the G sensor value becomes a negative value when the vehicle stopson a sloped road directed uphill and becomes a positive value when thevehicle is on a sloped road directed downhill.

The brake ECU 60 includes a CPU, a ROM and a RAM or the like (noneshown). Various kinds of control processing (braking control processingand the like shown in FIG. 8 for example), various kinds of maps (mapsand the like shown in FIGS. 6 and 7), and various threshold values arestored in advance in the ROM. The RAM stores various kinds ofinformation (wheel speed, G sensor value and the like) that areappropriately rewritten when an ignition switch (not shown) of thevehicle is ON. The brake ECU 60 can communicate with other ECUs 13 and23 of the vehicle through a bus 61.

The braking device 30 of the present embodiment assists the increasingof the braking force applied to the wheels FR, FL, RR, and RL when thecurrent depression operation performed on the brake pedal 31 by thedriver is an emergency braking operation. However, the braking device 30of the present embodiment does not include a sensor that directlydetects the depression force applied to the brake pedal 31 by the driver(e.g., pressure sensor for detecting MC pressure). Hence, in the presentembodiment, instead of a pressure sensor, the wheel speed sensors SE2 toSE5 and the vehicle body acceleration sensor SE6 are used to determinewhether the current depression operation of the brake pedal 31 performedby the driver is an emergency braking operation.

Next, a braking control method using the wheel speed sensors SE2 to SE5and the vehicle body acceleration sensor SE6 will be described withreference to the timing chart shown in FIG. 3.

As shown in FIG. 3, if a driver starts depressing the brake pedal 31 ata first timing t1, the MC pressure in the master cylinder 321 startsincreasing. The WC pressure in the wheel cylinders 55 a to 55 d alsostarts to increase following the increase in the MC pressure. As aresult, a braking force with a magnitude corresponding to the WCpressure is applied to the wheels FR, FL, RR, and RL. This suddenlydecreases the wheel speed VW of each of the wheels FR, FL, RR, and RL,that is, the moving speed of the wheel relative to the road surface.When the wheel speed VW decreases in this manner, the vehicle body speedVS starts to decrease. In the following description, the “wheel speedVW” refers to a value obtained using a detection signal of each of thewheel speed sensors SE2 to SE5, that is, a signal indicative of thewheel rotation speed.

As a result, the vehicle body deceleration (first estimated vehicle bodydeceleration) DV calculated using the detection signal of at least oneof the wheel speed sensors SE2 to SE5 starts to increase. Slightly afterthe vehicle body deceleration DV increases, the G sensor value (secondestimated vehicle body deceleration) G, which is calculated using adetection signal of the vehicle body acceleration sensor SE6, starts toincrease. The wheel speed sensors SE2 to SE5 are arranged at positionsin the vicinity of the wheels FR, FL, RR, and RL, whereas the vehiclebody acceleration sensor SE6 is separated from the wheels FR, FL, RR,and RL. Specifically, the vehicle body acceleration sensor SE6 isarranged on a vehicle body (not shown), which is supported by thesuspension (not shown) of the vehicle. Hence, when a braking force isapplied to the wheels FR, FL, RR, and RL, the G sensor value G starts tochange slightly after the vehicle body deceleration DV.

When the vehicle body deceleration DV exceeds a first decelerationdetermination value DV_st, which is set to be greater than a brakingdetermination value KDV_Brk, before the time elapsed from the secondtiming t2, at which the vehicle body deceleration DV exceeds the brakingdetermination value KDV_Brk, exceeds first reference elapsed time TDVst,a first initiation determination condition is satisfied (third timingt3). Subsequently, when the G sensor value G exceeds a seconddeceleration determination value G_st before the time elapsed from whenthe third timing t3 exceeds a second reference elapsed time TGst (e.g.,102 milliseconds), a second initiation determination condition issatisfied (fourth timing t4). If the first and second initiationdetermination conditions are satisfied, it is determined that thecurrent driver's depression operation on the brake pedal 31 is anemergency braking operation.

As a result, an assist control condition satisfaction flag FLG4 is setfrom OFF to ON, and the assist control (also referred to as “brakeassist control” and “BA control”) for assisting to increase a brakingforce applied to the wheels FR, FL, RR, and RL is started. The assistcontrol condition satisfaction flag FLG4 is set to ON from establishmentof an initiating condition of the assist control to establishment of atermination condition of the assist control.

The assist control includes an increasing control, which increases theWC pressure in each of the wheel cylinders 55 a to 55 d to increase thebraking force applied to the wheels FR, FL, RR, and RL, and a holdingcontrol, which holds the WC pressure to hold the braking force appliedto the wheels FR, FL, RR, and RL. In the increasing control, the linearsolenoid valve 44 and the pump 49 (motor 48) are operated (see FIG. 2).If the increasing control is performed during a preset increaserequisition time, the control is shifted to the holding control. In theholding control, the pump 49 is stopped, and the WC pressure in each ofthe wheel cylinders 55 a to 55 d is held by the operation of the linearsolenoid valve 44. When a driver's depression operation amount of thebrake pedal 31 is changed, the WC pressure in the wheel cylinders 55 ato 55 d is increased or decrease in correspondence with the change.

Then, when the depression operation amount of the brake pedal 31 becomessmall or the brake pedal 31 is released from the depressed state, thetermination condition of the assist control is satisfied, the assistcontrol condition satisfaction flag FLG4 is set to OFF, and the assistcontrol is terminated. That is, the supply of power to the linearsolenoid valve 44 is stopped, and the braking force applied to thewheels FR, FL, RR, and RL is reduced. An example of the terminationcondition is the G sensor value G becoming less than the seconddeceleration determination value G_st.

The above-described braking control method has the problems describedbelow.

A first problem is that among the vehicle body deceleration DV, which iscalculated using the detection signals of the wheel speed sensors SE2 toSE5, and the G sensor value G, which is calculated using a detectionsignal of the vehicle body acceleration sensor SE6, in particular, thevehicle body deceleration DV includes a vibration component resultingfrom outer disturbance. Examples of the outer disturbance includereaction forces received by the wheels FR, FL, RR, and RL from thesurface of the road along which the vehicle travels and interferencebetween the drive force transmitted to the front wheels FR and FL, whichare the drive wheels, and the braking force applied to the front wheelsFR and FL.

The magnitude of the reaction force received by each of the wheels FR,FL, RR, and RL from a road surface changes between a bad road having anuneven surface and a good road having an even surface. Moreover, evenwith a bad road, the magnitude of a vibration component in the vehiclebody deceleration DV changes in accordance with how uneven the road is.Further, when a vehicle moves over a bump on a road, a vibrationcomponent generated when the vehicle moves over the bump is included inthe vehicle body deceleration DV. To solve such a problem, it ispreferred that the degree of unevenness of the road surface beestimated, the movement of the vehicle over a bump be estimated, and thefirst deceleration determination value DV_st be corrected based on theestimation result.

The interference between the drive force and braking force applied tothe wheels FR, FL, RR, and RL may occur when the automatic transmission21 is downshifted during a braking operation. When a gear is downshiftedin the automatic transmission 21 (e.g., shifted from fourth gear tothird gear), the drive force transmitted to the front wheels FR and FLbecomes greater than just before the downshifting. As a result,interference occurs between the drive force and braking force applied tothe front wheels FR and FL. A vibration component caused by theinterference is included in the vehicle body deceleration DV. Hence,when a gear is downshifted in the automatic transmission 21 or whenthere is a high probability that a gear is downshifted in the automatictransmission 21, it is preferred that the first decelerationdetermination value DV_st be set as a large value.

A vibration component caused by outer disturbance is not included in theG sensor value G as much as the vehicle body deceleration DV. This isbecause the suspension (not shown) that supports the vehicle bodyfunctions as a damper.

A second problem is that detection signals from the wheel speed sensorsSE2 to SE5 are easily affected by the gradient of the road surface. Asshown in the timing chart of FIG. 4, when a road surface on which thevehicle is traveling is a sloped road, a difference that corresponds tothe gradient of the road surface is produced between the vehicle bodydeceleration DV and the G sensor value G. That is, when the road is asloped road directed uphill, gravity applied to the vehicle acts on thevehicle body as a braking force, and a component of the braking force isincluded in the vehicle body deceleration DV. When the road is a slopedroad directed downhill, gravity applied to the vehicle acts on thevehicle body as a propulsive force, and a component of the propulsiveforce is included in the vehicle body deceleration DV. Hence, unless thefirst deceleration determination value DV_st is corrected in accordancewith the gradient of the road surface, the vehicle body deceleration DVeasily exceeds the first deceleration determination value DV_st when theroad is a sloped road directed uphill, and the vehicle body decelerationDV is less prone to exceed the first deceleration determination valueDV_st when the road is a sloped road directed downhill.

The G sensor value G is deviated from the vehicle body deceleration DVby an amount corresponding to the gradient of a road surface. That is,the G sensor value G becomes less than the vehicle body deceleration DVwhen the road is a sloped road directed uphill, and the G sensor value Gbecomes greater than the vehicle body deceleration DV when the road is asloped road directed downhill. In other words, the timing for initiatingassist control varied depending on the gradient of a road surface. Tosuppress such variations in the initiating timing, it is preferred thatthe first deceleration determination value DV_st and the seconddeceleration determination value G_st be corrected based on the gradientof the road surface.

When the gradient of a road surface on which the vehicle travels changesand indicates an uphill slope, the braking force resulting from thechange in the gradient of the road surface is applied to the frontwheels FR and FL as shown in the timing chart of FIG. 5. That is, whenonly the front wheels FR and FL of the wheels FR, FL, RR, and RL pass bya changing point A of the gradient, gravity acting on the vehicle bodyis applied to the front wheels FR and FL as a braking force (firsttiming t21). This suddenly decreases the wheel speed VW of the frontwheels FR and FL, and the vehicle body deceleration DV suddenlyincreases. At the same time, the suspension (not shown) absorbs thechange in the gradient of the road surface. Thus, the G sensor value Gdoes not change as much as the vehicle body deceleration DV. Thus, thegradient change of the vehicle body deceleration DV differs greatly fromthe gradient change of the G sensor value G.

Then, when the rear wheels RR and RL also pass by the changing point Aof the gradient, the difference between the vehicle body deceleration DVand the G sensor value G approaches the difference corresponding to thegradient of the road surface (second timing t22). That is, unless thefirst deceleration determination value DV_st is corrected, the vehiclebody deceleration DV may easily exceed the first decelerationdetermination value DV_st between the first timing t21 and the secondtiming t22 thereby unintentionally satisfying the initiating conditionof the assist control. Hence, when the gradient of a road surfacechanges and indicates an uphill slope, it is preferred that the firstdeceleration determination value DV_st be corrected to a larger value.

Depending on the driver's depression operation on the brake pedal 31,the necessity for the assist control may be null or low. A highlyskilled driver who is good at driving the vehicle can appropriatelydepress the brake pedal 31 when necessary. More specifically, whenemergency braking is necessary, a highly skilled driver can readily andstrongly depress the brake pedal 31. In this case, a sufficiently largebraking force can be readily applied to the wheels FR, FL, RR, and RLjust by the driver's depression operation on the brake pedal 31. Thus,assist control is unnecessary. Hence, based on the detection signalsfrom the wheel speed sensors SE2 to SE5 and the vehicle bodyacceleration sensor SE6, when determining that the driver is applying alarge depression force to the brake pedal 31, it is preferred that theassist control not be performed.

When the initiating condition of the assist control is satisfied andimmediately after the increasing control of the assist control isinitiated, anti-lock braking control (also referred to as “ABS control”,hereinafter) that prevents locking of the wheels FR, FL, RR, and RL fromlocking may be initiated. Immediately after the increasing control isinitiated, the driver's depression operation of the brake pedal 31 mayinitiate the ABS control. Immediately after the increasing control ofthe assist control is initiated, the vehicle body deceleration DV maybecome greater than or equal to the deceleration corresponding to theroad surface limit (e.g., 1.2 G). In this case, if the braking forceapplied to the wheels FR, FL, RR, and RL is further increased, thepossibility of the ABS control being initiated is high. In such cases, asufficiently large braking force may be applied to the wheels FR, FL,RR, and RL just by the driver's depression operation on the brake pedal31. Hence, it is preferred that the assist control be terminated.

The ABS control may be initiated during execution of the holding controlof the assist control. Such a case may occur when the road surface alongwhich the vehicle is traveling is switched from a high p road to a low proad and the slip rates of the wheels FR, FL, RR, and RL become high. Ifthe ABS control is initiated during execution of the holding control,the braking force applied to the wheels FR, FL, RR, and RL is reduced tolower the slip rates of the wheels FR, FL, RR, and RL. Then, as thebraking force applied to the wheels FR, FL, RR, and RL decreases, thevehicle body deceleration DV and the G sensor value G also decrease. Inthis case, the driver most likely does not intend to perform a brakingoperation that reduces the braking force applied to the wheels FR, FL,RR, and RL. Thus, it is preferred that the assist control not beterminated. Further, after the ABS control is initiated, the roadsurface may be switched again from a low p road to a high p road. Insuch a case, if a driver's depression operation amount of the brakepedal 31 remains large, the assist control should be continued. In thismanner, when the ABS control is initiated during execution of theholding control, it is preferred that the holding control be continued.

Even if the driver's depression operation amount of the brake pedal 31is reduced during execution of the assist control, the assist controlmay not be terminated. This is because the G sensor value G may begreater than or equal to the second deceleration determination valueG_st due to just the braking force applied to the wheels FR, FL, RR, andRL based on execution of the assist control. Hence, the threshold valueused when determination termination of the assist control should takeinto consideration the braking force applied to the wheels FR, FL, RR,and RL by the execution of the assist control.

The brake ECU 60 of the present embodiment sets the starting timing andterminating timing of the assist control taking the above-describedmatters into consideration. Next, a map required for the brake ECU 60 toset the timing for initiating the assist control will be described withreference to FIGS. 6 and 7.

A first map shown in FIG. 6 will now be described.

The first map is used to correct the first deceleration determinationvalue DV_st when it determined that there is a possibility that theautomatic transmission 21 will be downshifted. The horizontal axis ofthe first map shows a third determination timer T3 corresponding to theduration time of a state in which the G sensor value G exceeds adownshift determination value, which is set to determine whether thereis a possibility that the automatic transmission 21 will be downshifted.The vertical axis of the first map shows a downshift determinationcorrection value DVflat, which is the correction amount of the firstdeceleration determination value DV_st. As shown in FIG. 6, when thethird determination timer T3 is less than or equal to the first timeT3_1 (e.g., 14), the downshift determination correction value DVflat isset to “0 (zero)”, and when the third determination timer T3 is greaterthan or equal to second time T3_2 (e.g., 50), which is longer than thefirst time T3_1, the downshift determination correction value DVflat isset to a maximum correction value KDVflat1 (e.g., 0.5 G). When the thirddetermination timer T3 is greater than the first time T3_1 and less thanthe second time T3_2, the downshift determination correction valueDVflat is set to a larger value as the value of the third determinationtimer T3 increases.

Next, a second map shown in FIG. 7 will be described.

The second map is used to set a gradient change reference value KDGlowfor determining whether or not the gradient of the road surface alongwhich the vehicle is traveling has changed and indicates an uphillslope. Here, the expression “gradient of a road surface is changed andindicates an uphill slope” refers to an increase in the gradient of theroad surface along which the vehicle is traveling. This expressionincludes a case in which when the gradient is a negative value, theabsolute value of the gradient becomes small, that is, the gradient ofthe downhill slope becomes gradual.

As shown in FIG. 7, the horizontal axis of the second map shows asubtraction value (=DDV−DG) obtained by subtracting a gradient change DGof the G sensor value G from a gradient change DDV of the vehicle bodydeceleration DV, and the vertical axis of the second map shows thegradient change reference value KDGlow. When the subtraction value(=DDV−DG) is less than or equal to a first deceleration value D1 (e.g.,0.3 G), the gradient change reference value KDGlow is set to a firstreference value KDGlow1 (e.g., 2 G/s). When the subtraction value isgreater than or equal to a second subtraction value D2 (e.g., 0.5 G),which is greater than the first deceleration value D1, the gradientchange reference value KDGlow is set to a second reference value KDGlow2(e.g., 4 G/s), which is greater than the first reference value KDGlow1.When the subtraction value is greater than the first deceleration valueD1 and less than the second subtraction value D2, the gradient changereference value KDGlow is set to a larger value as the subtraction valueincreases.

The gradient change DDV of the vehicle body deceleration indicates achange amount of the vehicle body deceleration DV per unit time and isacquired by time-differentiating the vehicle body deceleration DV, forexample. In the same manner, the gradient change DG of the G sensorvalue indicates a change amount of the G sensor value G per unit timeand is acquired by time-differentiating the G sensor value G, forexample.

Next, various kinds of control processing routines executed by the brakeECU 60 of the present embodiment will be described with reference toflowcharts shown in FIGS. 8 to 14. FIG. 8 shows a braking controlprocessing routine mainly executed by the brake ECU 60.

The braking control processing routine is executed in predetermined timecycles (e.g., 6 milliseconds) that are determined in advance. In thebraking control processing routine, the brake ECU 60 performsinformation acquisition processing for acquiring various kinds ofinformation (such as wheel speed) necessary when assist control oranti-lock braking control is performed (step S10). Subsequently, thebrake ECU 60 performs bad road determination processing for acquiring abad road index that numerically indicates the unevenness degree of theroad surface along which the vehicle is traveling (step S11).

The brake ECU 60 performs ABS determination processing to determinewhether or not the initiating condition of the ABS control is satisfied(step S12). Specifically, when the brake switch SW1 is ON, the brake ECU60 determines whether a slip rate of each of the wheels FR, FL, RR, andRL is greater than or equal to a preset slip rate determination valueand completes the ABS determination processing. The slip rate is a valuecalculated by the information acquisition processing. The calculation ofthe slip rate will be described later (see step S22 in FIG. 9).

Then, the brake ECU 60 performs ABS processing to prevent locking of thewheel (e.g., right front wheel FR) of which slip rate becomes greaterthan or equal to a slip rate determination value (step S13).Specifically, the brake ECU 60 repeatedly and sequentially performs adecreasing control for decreasing the braking force applied to the wheelthat is subject to control (e.g., right front wheel FR) and anincreasing control for increasing the braking force (and a holdingcontrol for holding the braking force). At this time, the brake ECU 60operates the pump 49, the pressure decreasing valve and the pressureincreasing valve for the wheel that is subject to control. Therefore, inthe present embodiment, the brake ECU 60 functions as an ABS controlunit.

The brake ECU 60 performs BA initiation determination processing fordetermining whether or not the initiating condition of the assistcontrol is satisfied (step S14). When satisfied, the brake ECU 60performs BA processing to execute the assist control (step S15).Subsequently, the brake ECU 60 determines whether or not the terminationcondition of the assist control is satisfied. When satisfied, the brakeECU 60 performs BA termination determination processing for terminatingthe assist control (step S16) and temporarily terminates the brakingcontrol processing routine.

Next, the information acquisition processing routine in step S10 will bedescribed with reference to the flowchart of FIG. 9.

In the information acquisition processing routine, the brake ECU 60calculates the wheel speeds VW of the wheels FR, FL, RR, and RL based onthe detection signals from the wheel speed sensors SE2 to SE5 (stepS20). Subsequently, the brake ECU 60 calculates the vehicle body speedVS (also referred to as the “estimated vehicle body speed”) based on thecalculated wheel speeds VW of at least one of the wheels FR, FL, RR, andRL (step S21). For example, the brake ECU 60 calculates a vehicle bodyspeed VS based on the wheel speeds VW of the rear wheels RR and RL,which are the driven wheels when a braking operation is not performed,and calculates the vehicle body speeds VS based on the wheel speeds VWof the wheels including the front wheels FR and FL, which are the drivewheels when the braking operation is performed. Accordingly, in thepresent embodiment, the brake ECU 60 also functions as a vehicle bodyspeed calculation unit. Then, the brake ECU 60 calculates slip rates SLP(=(VS−VW)/VW)) of the wheels FR, FL, RR, and RL (step S22).Subsequently, the brake ECU 60 calculates the vehicle body decelerationDV of the vehicle based on the vehicle body speed VS calculated in stepS21 (step S23). Accordingly, in the present embodiment, the brake ECU 60also functions as a first deceleration calculation unit that calculatesthe vehicle body deceleration (first estimated vehicle bodydeceleration) DV of the vehicle using detection signals of the wheelspeed sensors SE2 to SE5. Further, step S23 corresponds to a firstdeceleration calculating step. The vehicle body deceleration DV becomesa positive value when the vehicle is decelerated and becomes a negativevalue when the vehicle is accelerated.

Then, the brake ECU 60 acquires a gradient change DDV of the vehiclebody deceleration DV calculated in step S23 (step S24). Subsequently,the brake ECU 60 performs filtering processing to remove a highfrequency change component from the vehicle body deceleration DVcalculated in step S23 and acquires a leveled vehicle body decelerationDVf1 (step S25). As shown in FIG. 15, the brake ECU 60 performs afiltering processing to remove a low frequency change component from thevehicle body deceleration DV calculated in step S23 and acquires a noisecomponent DVf2 (step S26). The noise component DVf2 is used when a badroad index is acquired.

Thereafter, returning to the flowchart of FIG. 9, the brake ECU 60calculates the G sensor value G based on a detection signal from thevehicle body acceleration sensor SE6 (step S27). Accordingly, in thepresent embodiment, the brake ECU 60 also functions as a seconddeceleration calculation unit that calculates the G sensor value (secondestimated vehicle body deceleration) G of the vehicle with the vehiclebody acceleration sensor SE6. Step S27 corresponds to a seconddeceleration calculating step. Then, the brake ECU 60 calculates, asgradient information, the gradient change DG of the G sensor value Gcalculated in step S27 (step S28) and terminates the informationacquisition processing routine. Accordingly, in the present embodiment,the brake ECU 60 also functions as a gradient information acquisitionunit for calculating, as gradient information, the gradient change DG ofthe G sensor value (second estimated vehicle body deceleration). Thebrake ECU 60 also functions as a gradient change acquisition unit, whichacquires the gradient change DDV of the vehicle body deceleration andthe gradient change DG of the G sensor value.

Next, a bad road determination processing routine of step S11 will bedescribed with reference to the timing chart of FIG. 15.

In the bad road determination processing routine, the brake ECU 60acquires the noise component DVf2 from a predetermined number of samplescalculated in step S26 and calculates a dispersion value of the noisecomponent DVf2. The dispersion value of the noise components DVf2 is avalue obtained by squaring the noise component DVf2, integrating thesquared value, and dividing the integrated value by the number ofsamples. When the dispersion value is less than a preset firstdispersion threshold value, the brake ECU 60 sets a bad road index to “0(zero)”. When the dispersion value is greater than or equal to the firstdispersion threshold value and less than a preset second dispersionthreshold value, which is preset to be greater than the first dispersionthreshold value, the brake ECU 60 sets the bad road index to “1”. Whenthe dispersion value is greater than or equal to the second dispersionthreshold value and less than a third dispersion threshold value, whichis preset to be greater than the second dispersion threshold value, thebrake ECU 60 sets the bad road index to “2”, and when the dispersionvalue is greater than or equal to the third dispersion threshold value,the brake ECU 60 sets the bad road index to “3”. The dispersionthreshold values are used to set the bad road index of “0 (zero)” to “3”in accordance with the magnitude of the dispersion value and are presetby an experiment or a simulation. In this manner, as the unevennessdegree of a road surface increases, the bad road index is set at agreater value. Therefore, in the present embodiment, the brake ECU 60also functions as a bad road index acquisition unit. When the bad roadindex is “0 (zero)”, it is determined that the road surface is good, orthat the road is not a bad road.

Next, a BA initiation determination processing routine of step S14 willbe described with reference to the flowcharts shown in FIGS. 10, 11 and12 and the timing charts shown in FIGS. 5, 16, 17, 18 and 19.

The BA initiation determination processing routine is executed when thebrake switch SW1 is ON. In the BA initiation determination processingroutine, the brake ECU 60 determines whether or not the bad road indexNrw acquired in step S11 is “0 (zero)” (step S30). Accordingly, in thepresent embodiment, the brake ECU 60 also functions as an outerdisturbance determination unit that determines whether a vibrationcomponent based on outer disturbance (i.e., bad road) is included in thevehicle body deceleration DV. Step 30 corresponds to an outerdisturbance determining step. When the bad road index Nrw is “0 (zero)”(step S30: YES) the brake ECU 60 determines that the road surface isgood, and sets a bad road determination correction value DVbad to “0(zero)” (step S31). The bad road determination correction value DVbad isused to correct the first deceleration determination value DV_st.Subsequently, the brake ECU 60 sets a bad road correction flag FLG1 toOFF (step S32) and proceeds to step S39, which will be described later.

If the bad road index Nrw is not “0 (zero)” (step S30: NO), the brakeECU 60 determines whether the bad road index Nrw is “1” (step S33). Whenthe bad road index Nrw is “1” (step S33: YES), the brake ECU 60determines that the road surface is a slightly bad and sets the bad roaddetermination correction value DVbad to a first correction value KDVbad1(e.g., 0.2 G) (step S34). Subsequently, the brake ECU 60 sets the badroad correction flag FLG1 to ON (step S35) and proceeds to step S39,which will be described later.

When the bad road index Nrw is not “1” (step S33: NO), the brake ECU 60determines whether the bad road index Nrw is “2” (step S36). When thebad road index Nrw is “2” (step S36: YES), the brake ECU 60 determinesthat the road surface is a normal bad road and sets the bad roaddetermination correction value DVbad to a second correction valueKDVbad2 (e.g., 0.4 G), which is greater than the first correction valueKDVbad1 (step S37). Subsequently, the brake ECU 60 proceeds to step S35,which is described above. When the bad road index Nrw is not “2” (stepS36: NO), the bad road index Nrw is “3”. Thus, the brake ECU 60determines that the road surface is an extremely bad road and sets thebad road determination correction value DVbad to a third correctionvalue KDVbad3 (e.g., 0.6 G), which is greater than the second correctionvalue KDVbad2 (step S38). Subsequently, the brake ECU 60 proceeds tostep S35. In the present embodiment, a larger unevenness degree of theroad surface, or a larger bad road index Nrw, sets a larger bad roaddetermination correction value DVbad.

In step S39, the brake ECU 60 increments a first determination timer T1by “1”. The brake ECU 60 determines whether or not the firstdetermination timer T1 has exceeded a first time determination value T1th (e.g., 67) (step S40). The braking control processing routine isexecuted in predetermined time cycles (e.g., 6 milliseconds). Hence,step S40 is performed to determine whether the time corresponding to thefirst determination timer T1 exceeds a determination time T400 (e.g.,402 milliseconds), which is obtained by multiplying the first timedetermination value T1 th by a predetermined time (see FIG. 4). When thefirst determination timer T1 is less than or equal to the first timedetermination value T1 th (step S40: NO), the brake ECU 60 acquires, asa first difference value DVsub1, an absolute value of the differencebetween the vehicle body deceleration DV calculated in step S23 and theleveled vehicle body deceleration DVf1 calculated in step S25 (stepS41).

Subsequently, the brake ECU 60 determines whether an integrationpermission flag FLGs is OFF and whether the previous difference valueDVsub1 b is greater than the current first difference value DVsub1 (stepS421). The integration permission flag FLGs is used to integrate the topvalue (or value close to the top value) of the first difference valueDVsub1, which changes in a substantially cyclic manner. The previousdifference value DVsub1 b is a first difference value DVsub1 calculatedat the previous timing.

When the integration permission flag FLGs is OFF and the previousdifference value DVsub1 b is greater than the current first differencevalue DVsub1 (step S421: YES), the brake ECU 60 proceeds to step S422.In step S422, the brake ECU 60 integrates the first difference valueDVsub1 calculated in step S41 with an amplitude integration value σwDVof the vehicle body deceleration, increments the number CT1 ofintegrating operations by “1”, and sets the integration permission flagFLGs to ON (step S422). Subsequently, the brake ECU 60 subtracts the Gsensor value G calculated in step S27 from the leveled vehicle bodydeceleration DVf1, acquires the second difference value DVsub2 (stepS43), and integrates the second difference value DVsub2 calculated instep S43 with a gradient integration value σsG (step S441). The brakeECU 60 sets the previous difference value DVsub1 b as the current firstdifference value DVsub1 (step S442) and proceeds to step S50, which willbe described later.

When at least one of a condition that the integration permission flagFLGs is OFF and a condition that the previous difference value DVsub1 bis greater than the current first difference value DVsub1 is notsatisfied (step S421: NO), the brake ECU 60 determines whether theprevious difference value DVsub1 b is less than or equal to the currentfirst difference value DVsub1 (step S423). When the previous differencevalue DVsub1 b is greater than the current first difference value DVsub1(step S423: NO), the brake ECU 60 proceeds to step S43, which isdescribed above. When the previous difference value DVsub1 b is lessthan or equal to the current first difference value DVsub1 (step S423:YES), the brake ECU 60 sets the integration permission flag FLGs to OFF(step S424) and proceeds to step S43, which is described above.

When the first determination timer T1 exceeds the first timedetermination value T1 th (step S40: YES), the brake ECU 60 divides theacquired amplitude integration value σwDV of the vehicle bodydeceleration by the renewed number CT1 of integration operations untilthe first determination timer T1 exceeds the first time determinationvalue T1 th and acquires an amplitude W_DV of the vehicle bodydeceleration (step S45). The amplitude W_DV is used when the firstdeceleration determination value DV_st is corrected. Accordingly, in thepresent embodiment, the brake ECU 60 also functions as an amplitudecalculator.

Subsequently, the brake ECU 60 determines whether the calculatedamplitude W_DV is less than a preset amplitude reference value KW (stepS46). Detection signals from the wheel speed sensors SE2 to SE5 used foracquiring the vehicle body deceleration DV include a certain amount offluctuation (i.e., slight cyclic fluctuation) even if there is no outerdisturbance as described above. There is no need to correct the firstdeceleration determination value DV_st based on such a slight cyclicfluctuation. When the road surface along which the vehicle is travelingis a so-called gravel road over which gravel is spread, the bad roadindex Nrw often becomes “0 (zero)”. In such a case, even if it isdetermined that the road surface is good, the vibration component basedon slight unevenness of the road surface is included in the vehicle bodydeceleration DV. Hence, in the present embodiment, the amplitudereference value KW is set as a determination value used to determinewhether the influence of outer disturbance is included in the vehiclebody deceleration DV.

When the amplitude W_DV is less than the amplitude reference value KW(step S46: YES), the brake ECU 60 sets the amplitude W_DV to “0 (zero)”(step S47), and proceeds to step S48. If the amplitude W_DV is greaterthan or equal to the amplitude reference value KW (step S46: NO), thebrake ECU 60 proceeds to step S48 without performing step S47.

In step S48, the brake ECU 60 divides the acquired gradient integrationvalue σsG by the first determination timer T1 and acquires a gradientestimated value (gradient information) Gslope. The gradient estimatedvalue Gslope is the gradient of the road surface and used when the firstdeceleration determination value DV_st and the second decelerationdetermination value G_st are corrected. Accordingly, in the presentembodiment, the brake ECU 60 also functions as a gradient informationacquisition unit, which acquires the gradient estimated value Gslope asgradient information. Step S48 corresponds to a gradient informationacquiring step. Subsequently, the brake ECU 60 sets the firstdetermination timer T1, the number CT1 of the integrating operations,the amplitude integration value σwDV of the vehicle body deceleration,and the gradient integration value σsG to “0 (zero)” (step S49) and thenproceeds to following step S50.

More specifically, in the timing chart shown in FIG. 4, the differencevalues DVsub1 and DVsub2 are acquired in predetermined time cycles fromthe first timing t11, at which the acquisition operation of thedifference values DVsub1 and DVsub2 is started, to a second timing t12,at which determination time T400 elapses from the first timing t11. Thegradient estimated value Gslope used to correct the decelerationdetermination values DV_st and G_st is set to an average value of thesecond difference values DVsub2 acquired during the determination timeT400. The amplitude W_DV for correcting the deceleration determinationvalue DV_st is set based on the first difference value DVsub1 acquiredduring the determination time T400. That is, the amplitude W_DV and thegradient estimated value Gslope are renewed for every determination timeT400.

Returning to the flowchart shown in FIG. 10, the brake ECU 60 determinesin step S50 whether or not the vehicle is moving over a bump on a roadsurface. A state in which wheel speeds VW of the wheels FR, FL, RR, andRL change when the vehicle moves over a bump will now be described withreference to the timing chart shown in FIG. 16. When the vehicle movesover a bump, the front wheels FR and FL first move over the bump. Inthis state, the wheel speeds VW of the front wheels FR and FL aresuddenly decelerated because the front wheels FR and FL come intocontact with the bump (first timing t31). Then, the center of gravity ofthe vehicle is vertically changed because the front wheels FR and FLcome into contact with the bump. As a result, the wheel speeds VW of thefront wheels FR and FL are changed in accordance with the verticalchange in the center of gravity. That is, when the center of gravity ofthe vehicle is vertically moved, the contact area between the frontwheels FR and FL and the road surface becomes small. Hence, the tractionforce between the front wheels FR and FL and the road surface becomessmall, and the wheel speeds VW of the front wheels FR and FL areaccelerated. If the wheel speeds VW of the front wheels FR and FL startaccelerating in this manner, the vehicle body deceleration DV starts todecrease (second timing t32).

Then, when the center of gravity of the vehicle starts moving downwardand the contact area between the road surface and the front wheels FRand FL increases, the traction force between the road surface and thefront wheels FR and FL increases and the wheel speeds VW of the frontwheels FR and FL start to decelerate. From the third timing t33, atwhich a gradient change of the vehicle body deceleration DV becomesgradual, the vehicle body deceleration DV becomes less than the G sensorvalue G. Hence, after the third timing t33, the vehicle bodydeceleration DV becomes greater toward the G sensor value G in recoilfrom the preceding state. As shown at the fourth timing t34, the vehiclebody deceleration DV may become greater than the G sensor value G. Inthis case, if the first deceleration determination value DV_st is notcorrected, the vehicle body deceleration DV becomes greater than orequal to the first deceleration determination value DV_st, and theassist control may start in an unintended manner. Hence, it ispreferable that the first deceleration determination value DV_st becorrected before the fourth timing t34.

When the driver's depression operation amount of the brake pedal 31 issmall, the vehicle body deceleration DV changes as shown in the timingchart of FIG. 17. That is, when the vehicle moves over a bump, the frontwheels FR and FL come into contact with the bump. This suddenlydecelerates the wheel speeds VW of the front wheels FR and FL (firsttiming t31-1). Then, the center of gravity of the vehicle is verticallychanged due to the contact of the front wheels FR and FL with the bump.As a result, the wheel speeds VW of the front wheels FR and FL arechanged in accordance with the change in the center of gravity in thevertical direction. That is, when the center of gravity of the vehicleis moved upward, the contact area between the road surface and the frontwheels FR and FL becomes small. Hence, the traction force between theroad surface and the front wheels FR and FL becomes small, and the wheelspeeds VW of the front wheels FR and FL are accelerated. If the wheelspeeds VW of the front wheels FR and FL start to accelerate, the vehiclebody deceleration DV starts to decrease.

In this state, if the vehicle body deceleration DV before the wheelspeeds VW of the front wheels FR and FL start to accelerate is small,the vehicle body deceleration DV immediately becomes a negative value(third timing t33-1). After the third timing t33-1, the vehicle bodydeceleration DV is increased toward the G sensor value G in recoil fromthe preceding state. In this case, if the first decelerationdetermination value DV_st is not corrected, the vehicle bodydeceleration DV becomes greater than or equal to the first decelerationdetermination value DV_st, and the assist control may start in anunintended manner. Thus, it is preferable that the first decelerationdetermination value DV_st be corrected at the third timing t33-1.

Returning back to the flowchart shown in FIG. 10, in the presentembodiment, the determination processing in step S50 is performed.Specifically, the brake ECU 60 determines whether one of the followingtwo conditions is satisfied.

(First Condition) A value (=G−DV) obtained by subtracting the vehiclebody deceleration DV from the G sensor value G exceeds a presetdeceleration specified value DVth1 (e.g., 0.2 G).

(Second Condition) The vehicle body deceleration DV is less than “0(zero)”.

More specifically, once the G sensor value G becomes greater than thetotal of the vehicle body deceleration DV and the deceleration specifiedvalue DVth1, the probability of the vehicle body deceleration DVsuddenly increasing afterward is high. If the vehicle body decelerationDV becomes a negative value, this indicates that the vehicle body speedVS has erroneously been determined as accelerating even though a brakingoperation is being performed. Hence, when one of the above conditions issatisfied, it is determined that the vehicle moved over a bump on aroad. Therefore, in the present embodiment, the brake ECU 60 alsofunctions as an outer disturbance determination unit that determineswhether or not a vibration component based on outer disturbance (i.e.,bump) is included in vehicle body deceleration (first estimated vehiclebody deceleration) DV. Step S50 corresponds to an outer disturbancedetermining step. A case in which the first condition is not satisfiedand the second condition is satisfied can occur when the vehicle movesover a bump in a state in which the driver's depression operation amountof the brake pedal 31 is small.

When both conditions are not satisfied (step S50: NO), the brake ECU 60determines that the vehicle has not moved over a bump and sets a seconddetermination timer T2 and a bump determination correction value DVstepto “0 (zero)” (step S51). The brake ECU 60 sets a bump correction flagFLG2 to OFF (step S52) and proceeds to step S57, which will be describedlater.

When the first or the second condition is satisfied (step S50: YES), thebrake ECU 60 determines that the vehicle moved over a bump andincrements the second determination timer T2 by “1” (step S53).Subsequently, the brake ECU 60 determines whether the seconddetermination timer T2 is less than or equal to a preset second timedetermination value T2 th (e.g., 34) (step S54). The braking controlprocessing routine is executed every predetermined time (e.g., 6milliseconds). Hence, it is determined in step S53 whether the timecorresponding to the second determination timer T2 has exceeded adetermination time T200 (e.g., 204 milliseconds) obtained by multiplyingthe second time determination value T2 th by a predetermined time (seeFIGS. 16 and 17). When the second determination timer T2 exceeds thesecond time determination value T2 th (step S54: NO), the brake ECU 60proceeds to step S51. That is, the brake ECU 60 sets the bumpdetermination correction value DVstep which is used to correct the firstdeceleration determination value DV_st to “0 (zero)”. Therefore, in thepresent embodiment, the second time determination value T2 thcorresponds to a deceleration specified time.

When the second determination timer T2 is less than or equal to thesecond time determination value T2 th (step S54: YES), the brake ECU 60sets the bump determination correction value DVstep to a thirdcorrection value KDVbad3 (e.g., 0.6 G) (step S55). Subsequently, thebrake ECU 60 sets the bump correction flag FLG2 to ON (step S56) andproceeds to following step S57.

In step S57, the brake ECU 60 determines whether it is possible tocommunicate with the AT ECU 23, which serves as another control unit(transmission control unit) that controls the automatic transmission 21.When it is not possible to communicate with the AT ECU 23 (step S57:NO), the brake ECU 60 proceeds to step S66, which will be describedlater. When it is possible to communicate with the AT ECU 23 (step S57:YES), the brake ECU 60 determines whether a downshift signal instructingthe execution of a downshifting operation has been received from the ATECU 23 (step S58). When the downshift signal has not yet been received(step S58: NO), the brake ECU 60 proceeds to step S61, which is willdescribed later. When the downshift signal has been received (step S58:YES), the brake ECU 60 sets, to the maximum correction value KDVflat1(e.g., 0.5 G), a downshift determination correction value DVflat that isused to correct the first deceleration determination value DV_st (stepS59). Accordingly, in the present embodiment, the brake ECU 60 alsofunctions as an outer disturbance determination unit that determineswhether or not a vibration component based on outer disturbanceresulting from the downshifting operation of the automatic transmission21 is included in the vehicle body deceleration (first estimated vehiclebody deceleration) DV. Step S58 corresponds to an outer disturbancedetermining step. Then, the brake ECU 60 sets a downshift flag FLGd toON (step S60) and shifts the processing to step S70, which will bedescribed later.

In step S61, the brake ECU 60 determines whether or not the downshiftflag FLGd is ON. If the downshift flag FLGd is OFF (step S61: NO), thebrake ECU 60 proceeds to step S70, which will be described later. Whenthe downshift flag FLGd is ON (step S61: YES), the brake ECU 60increments a downshift timer Td by “1” (step S62). The brake ECU 60determines whether the downshift timer Td exceeds a preset gear shiftingcompletion determination value KTd (e.g., 17) (step S63). The brakingcontrol processing routine is executed every predetermined time (e.g., 6milliseconds). Thus, it is determined in step S62 whether the timecorresponding to the downshift timer Td exceeds the determination time(e.g., 102 milliseconds) obtained by multiplying the gear shiftingcompletion determination value KTd by a predetermined time. Accordingly,in the present embodiment, the gear shifting completion determinationvalue KTd corresponds to a gear shifting specified time.

When the downshift timer Td is less than or equal to the gear shiftingcompletion determination value KTd (step S63: NO), the brake ECU 60proceeds to step S70, which will be described later. When the downshifttimer Td exceeds the gear shifting completion determination value KTd(step S63: YES), the brake ECU 60 sets the downshift determinationcorrection value DVflat to “0 (zero)” (step S64) and sets the downshiftflag FLGd to OFF (step S65). The brake ECU 60 proceeds to step S70,which will be described later.

A downshift determination value (high depression force determinationreference value) KGflat (e.g., 0.3 G) is set to determine whether adriver's depression operation amount of the brake pedal 31 is large. Instep S66, the brake ECU 60 determines whether the G sensor value Gcalculated in step S27 exceeds the downshift determination value KGflat.If the G sensor value G exceeds the downshift determination value KGflat(step S66: YES), the brake ECU 60 increments the third determinationtimer T3 by “1” (step S67). The third determination timer T3 correspondsto the duration time in a state in which the G sensor value G exceedsthe downshift determination value KGflat. Accordingly, in the presentembodiment, the brake ECU 60 also functions as a duration timeacquisition unit that acquires the third determination timer T3 as aduration time in a state in which the G sensor value (second estimatedvehicle body deceleration) G exceeds the downshift determination value(high depression force determination reference value) KGflat.

The brake ECU 60 sets the downshift determination correction valueDVflat to a value corresponding to the third determination timer T3using the first map (see FIG. 6) (step S68) and proceeds to step S70,which will be described later. More specifically, when the thirddetermination timer T3 is less than or equal to the first time T3_1, thedownshift determination correction value DVflat is set to “0 (zero)”.When the third determination timer T3 exceeds the first time T3_1, thedownshift determination correction value DVflat is set to a valuegreater than “0 (zero)”. That is, the first time T3_1 corresponds to ahigh depression force specified time. Accordingly, in the presentembodiment, the brake ECU 60 also functions as an outer disturbancedetermination unit which determines whether or not a vibration componentbased on outer disturbance resulting from a downshifting operation ofthe automatic transmission 21 may be included in the vehicle bodydeceleration (first estimated vehicle body deceleration) DV. Step S68corresponds to an outer disturbance determining step. When the G sensorvalue G is less than the downshift determination value KGflat (step S66:NO), the brake ECU 60 sets the third determination timer T3 and thedownshift determination correction value DVflat to “0 (zero)” (step S69)and proceeds to following step S70.

In step S70, the brake ECU 60 sets, using the second map (see FIG. 7),the gradient change reference value KDGlow used to determine whether ornot a road surface along which the vehicle is traveling has changed toan uphill slope. Specifically, the brake ECU 60 subtracts the gradientchange DG of the G sensor value calculated in step S28 from the gradientchange DDV of the vehicle body deceleration calculated in step S24 toset a value based on the subtraction result as the gradient changereference value KDGlow. Accordingly, in the present embodiment, thebrake ECU 60 also functions as a reference value setting unit that setsthe gradient change reference value KDGlow. Then, the brake ECU 60determines whether the gradient change DG of the G sensor valuecalculated in step S28 is less than the gradient change reference valueKDGlow, which is set in step S70 (step S71). When the gradient change DGis less than the gradient change reference value KDGlow (step S71: YES),the brake ECU 60 determines that the road surface has changed to anuphill road and sets a gradient change correction value DVDGlow, whichis used to correct the first deceleration determination value DV_st, toa preset maximum gradient correspondence value KDVDGlow (e.g., 0.45 G)(step S72). The maximum gradient correspondence value KDVDGlow is adeceleration component corresponding to the maximum value (e.g., 50%) ofa road surface gradient on which the vehicle can travel. Then, the brakeECU 60 proceeds to step S74, which will be described later. When thegradient change DG is greater than or equal to the gradient changereference value KDGlow (step S71: NO), the brake ECU 60 determines thatthe road surface has not changed to an uphill slope, sets the gradientchange correction value DVDGlow to “0 (zero)” (step S73), and shifts theprocessing to following step S74.

In step S74, the brake ECU 60 determines whether the bump correctionflag FLG2 is ON. When the bump correction flag FLG2 is ON (step S74:YES), the brake ECU 60 sets a deceleration correction value DVtp as thebump determination correction value DVstep set in step S55 (step S75)and proceeds to step S79. When the bump correction flag FLG2 is OFF(step S74: NO), the brake ECU 60 determines whether the bad roadcorrection flag FLG1 is ON (step S76). When the bad road correction flagFLG1 is ON (step S76: YES), the brake ECU 60 sets the decelerationcorrection value DVtp to a bad road determination correction value DVbadset in any one of steps S34, S37 and S38 and proceeds to step S79, whichwill be described later. When the bad road correction flag FLG1 is OFF(step S76: NO), the brake ECU 60 sets the deceleration correction valueDVtp to the amplitude W_DV, which is set in step S45 or S47, andproceeds to following step S79.

In step S79, the brake ECU 60 sets the first deceleration determinationvalue DV_st. Specifically, the brake ECU 60 adds the decelerationcorrection value DVtp, the gradient estimated value Gslope, thedownshift determination correction value DVflat and the gradient changecorrection value DVDGlow to a preset basic value KDV (e.g., 0.5 G).Then, the brake ECU 60 sets the added result as the first decelerationdetermination value DV_st. That is, when it is determined that thevehicle has moved over a bump, the first deceleration determinationvalue DV_st is set to a value that is greater than the basic value KDVby the bump determination correction value DVstep (see FIGS. 16 and 17).When determined that a road surface is a bad road, the firstdeceleration determination value DV_st is set to a value that is greaterthan the basic value KDV by a bad road determination correction valueDVbad. When it is determined that the road surface is a good road, thefirst deceleration determination value DV_st is set to a value greaterthan the basic value KDV by the amplitude W_DV. The first decelerationdetermination value DV_st is corrected based on the gradient of the roadsurface (see FIG. 4). When it is determined that the road surface haschanged to an uphill road, the first deceleration determination valueDV_st is corrected to a value greater than the basic value KDV by thegradient change correction value DVDGlow (see FIG. 5). When determinedthat the automatic transmission 21 has performed downshifting or thereis a possibility that the automatic transmission 21 has performeddownshifting, the first deceleration determination value DV_st iscorrected to a value that is greater than the basic value KDV by thedownshift determination correction value DVflat. Accordingly, in thepresent embodiment, the brake ECU 60 also functions as a reference valuecorrection unit. Step S79 corresponds to a reference value correctingstep.

The brake ECU 60 sets the second deceleration determination value G_st(step S80). Specifically, the brake ECU 60 subtracts the gradientestimated value Gslope from a preset basic value KGst (e.g., 0.3 G) andsets the subtracted result as the second deceleration determinationvalue G_st (see FIG. 4). The basic value KGst is preset as adetermination reference for determining whether a driver's operationamount of the brake pedal 31 has decreased during a period in whichassist control is not executed. Subsequently, the brake ECU 60determines whether the vehicle body deceleration DV calculated in stepS23 exceeds the braking determination value KDV_Brk (see FIG. 3) (stepS81). When the vehicle body deceleration DV is less than or equal to thebraking determination value KDV_Brk (step S81: NO), the brake ECU 60sets a fourth determination timer T4 and a fifth determination timer T5to “0 (zero)” and sets a first condition satisfaction flag FLG3 to OFF(step S82). Then, the brake ECU 60 proceeds to step S92, which will bedescribed later.

When the vehicle body deceleration DV exceeds the braking determinationvalue KDV_Brk (step S81: YES), the brake ECU 60 increments the fourthdetermination timer T4 by “1” (step S83). The fourth determination timerT4 corresponds to the time elapsed from when the vehicle bodydeceleration DV exceeds the braking determination value KDV_Brk.Subsequently, the brake ECU 60 determines whether the following twoconditions are both satisfied (step S84).

(Third Condition) The vehicle body deceleration DV exceeds the firstdeceleration determination value DV_st.

(Fourth Condition) The fourth determination timer T4 is less than orequal to an elapsed time determination value KT1 (e.g., 10).

The fourth condition can also be referred to as the time elapsed fromwhen the vehicle body deceleration DV exceeds the braking determinationvalue KDV_Brk being less than or equal to the first reference elapsedtime TDVst (see FIG. 3). That is, the first reference elapsed time TDVstis a value obtained by multiplying the elapsed time determination valueKT1 by a predetermined time (e.g., 6 milliseconds).

When the third and fourth conditions are both satisfied (step S84: YES),the brake ECU 60 determines whether the first condition satisfactionflag FLG3 is OFF (step S85). When the first condition satisfaction flagFLG3 is ON (step S85: NO), the brake ECU 60 shifts the processing tolater-described step S88. When the first condition satisfaction flagFLG3 is OFF (step S85: YES), the brake ECU 60 sets the gradient changeDDV of the current vehicle body deceleration as a first gradient changeDDV1 and sets the first condition satisfaction flag FLG3 to ON (stepS86). Then, the brake ECU 60 proceeds to step S88, which will bedescribed later. Therefore, in step S86 of the present embodiment, thegradient change DDV of the vehicle body deceleration acquired when thefirst condition satisfaction flag FLG3 is set from OFF to ON is acquiredas the first gradient change DDV1. In this aspect, step S86 correspondsto a first gradient acquiring step.

When at least one of the third and fourth conditions is not satisfied(step S84: NO), the brake ECU 60 determines whether the first conditionsatisfaction flag FLG3 is ON (step S87). When the first conditionsatisfaction flag FLG3 is OFF (step S87: NO), the brake ECU 60 proceedsto step S92, which will be described later. When the first conditionsatisfaction flag FLG3 is ON (step S87: YES), the brake ECU 60 proceedsto following step S88.

In step S88, the brake ECU 60 increments the fifth determination timerT5 by “1”. The fifth determination timer T5 corresponds to the timeelapsed from when the first condition satisfaction flag FLG3 is turnedON. Accordingly, in the present embodiment, the brake ECU 60 alsofunctions as an elapsed time acquisition unit that acquires the fifthdetermination timer T5 as the time elapsed from when the first gradientchange DDV1 is acquired. Subsequently, the brake ECU 60 determineswhether the following two conditions are both satisfied (step S89).

(Fifth Condition) The G sensor value G exceeds the second decelerationdetermination value G_st.

(Sixth Condition) The fifth determination timer T5 is greater thanspecified waiting time KT_w (e.g., 8) and less than or equal to aninitiation time determination reference value KT2 (e.g., 17).

The initiation time determination reference value KT2 corresponds to thesecond reference elapsed time TGst (see FIG. 3). That is, the secondreference elapsed time TGst is a value obtained by multiplying theinitiation time determination reference value KT2 by predetermined time(e.g., 6 milliseconds). In the present embodiment, the initiation timedetermination reference value KT2 is a specified value that is set basedon characteristics of a vehicle. The specified waiting time KT_wcorresponds to a depression force determination time reference value.

When at least one of the fifth and sixth conditions is not satisfied(step S89: NO), the brake ECU 60 proceeds to step S92, which will bedescribed later. When the fifth and sixth conditions are both satisfied(step S89: YES), the brake ECU 60 sets the gradient change DDV of thecurrent vehicle body deceleration as a second gradient change DDV2 anddetermines whether or not the second gradient change DDV2 is greaterthan or equal to the first gradient change DDV1 (step S90). Accordingly,in the present embodiment, step S90 corresponds to a second gradientacquisition step.

Here, a comparison between a case in which the depression force appliedby the driver to the brake pedal 31 is normal (low) and a case in whichthe depression force is high will be described with reference to thetiming charts shown in FIGS. 18 and 19. FIG. 19 shows a state in whichABS control is executed.

When the depression force is normal, as shown in the timing chart ofFIG. 18, the vehicle body deceleration DV is changed by a change in thebraking force applied to the wheels FR, FL, RR, and RL due to a recoilforce from the brake pedal 31, increased until the second timing t42,and decreased after the second timing t42. The second timing t42 is atiming when the fifth and sixth conditions are both satisfied. When thedepression force is normal, the probability is high that the secondgradient change DDV2 acquired at the second timing t42 will become lessthan the first gradient change DDV1 acquired at the first timing t41 atwhich the vehicle body deceleration DV exceeds the first decelerationdetermination value DV_st.

When the depression force is maintained at a high level, as shown in thetiming chart in FIG. 19, the depression force is sufficiently greaterthan the reaction force from the brake pedal 31. Thus, the vehicle bodydeceleration DV, which is changed by the change in the braking forceapplied to the wheels FR, FL, RR, and RL, takes a long time to decrease.Hence, when the depression force is maintained at a high level, theprobability is high that the second gradient change DDV2 acquired at thesecond timing t52, at which the fifth and sixth conditions aresatisfied, will become greater than or equal to the gradient change DDV(first gradient change DDV1) at the first timing t51 is high. Hence, inthis embodiment, when the second gradient change DDV2 is greater than orequal to the first gradient change DDV1, it is determined that adecrease in the necessity of the assist control increases the depressionforce applied by the driver to the brake pedal 31.

Returning to the flowchart shown in FIG. 12, when the second gradientchange DDV2 is greater than or equal to the first gradient change DDV1(step S90: YES), the brake ECU 60 determines that there is no need toexecute the assist control or the necessity of the assist control is lowand proceeds to step S92, which will be described later. Therefore, inthe present embodiment, the brake ECU 60 also functions as a depressionforce determination unit that determines whether or not the depressionforce when a driver depresses the brake pedal 31 is high. Step S90corresponds to a depression force determining step. When the secondgradient change DDV2 is less than the first gradient change DDV1 (stepS90: NO), the brake ECU 60 determines that it is necessary to executethe assist control, and sets the assist control condition satisfactionflag FLG4 to ON to indicate that the initiating condition of the assistcontrol has been satisfied (step S91). Then, the brake ECU 60 completesthe BA initiation determination processing routine.

In step S92, the brake ECU 60 sets the assist control conditionsatisfaction flag FLG4 to OFF and then completes the BA initiationdetermination processing routine.

Next, the BA processing routine in step S15 will be described withreference to the flowchart shown in FIG. 13.

In the BA processing routine, the brake ECU 60 determines whether theassist control condition satisfaction flag FLG4 is ON (step S100). Whenthe assist control condition satisfaction flag FLG4 is OFF (step S100:NO), the brake ECU 60 completes the BA processing routine withoutperforming the assist control. When the assist control conditionsatisfaction flag FLG4 is ON (step S100: YES), the brake ECU 60determines whether an increase completion flag FLG5 is OFF (step S101).When the increase completion flag FLG5 is ON (step S101: NO), the brakeECU 60 determines that the increasing control is completed and proceedsto step S107, which will be described later.

When the increase completion flag FLG5 is OFF (step S101: YES), thebrake ECU 60 increments a sixth determination timer T6 by “1” (stepS102). Subsequently, the brake ECU 60 determines whether of thedeceleration correction value DVtp, the downshift determinationcorrection value DVflat, and the gradient change correction valueDVDGlow are all “0 (zero)” (step S103). If of the correction valuesDVtp, DVflat, and DVDGlow are all “0 (zero)” (step S103: YES), the brakeECU 60 performs a first increasing control (step S104). The firstincreasing control increases the braking force applied to the wheels FR,FL, RR, and RL at a first increasing speed. The brake ECU 60 determineswhether the sixth determination timer T6 is greater than or equal tofirst determination time TBA1 th (step S105). The first determinationtime TBA1 th corresponds to an increase requisition time during whichthe assist control increases the braking force.

When the sixth determination timer T6 is less than the firstdetermination time TBA1 th (step S105: NO), the brake ECU 60 terminatesthe BA processing routine to continue the first increasing control. Whenthe sixth determination timer T6 is greater than or equal to the firstdetermination time TBA1 th (step S105: YES), the brake ECU 60 sets theincrease completion flag FLG5 to ON to indicate completion of theincreasing control (step S106). That is, in the present embodiment, thesixth determination timer T6 corresponds to the time elapsed from whenthe increasing control is initiated. Subsequently, the brake ECU 60performs the holding control for holding the braking force applied tothe wheels FR, FL, RR, and RL (step S107) and terminates the BAprocessing routine.

When at least one of the correction values DVtp, DVflat and DVDGlow isnot “0 (zero)” (step S103: YES), the first deceleration determinationvalue DV_st has been corrected due to outer disturbance or interference.Thus, the brake ECU 60 determines that there is a possibility that thecurrent assist control may have been executed in an unintended manner.Further, the brake ECU 60 performs a second increasing control in whichan increasing speed of the braking force applied to the wheels FR, FL,RR, and RL is set to a second increasing speed that is slower than thefirst increasing speed (step S108). For example, the second increasingspeed is about one half of the first increasing speed. In the secondincreasing control, the operating speed of the pump 49 may be decreasedor the moving speed of a valve body of the linear solenoid valve 44 maybe decreased as compared with the first increasing control.

Then, the brake ECU 60 determines whether the sixth determination timerT6 is greater than or equal to a second determination time TBA2 th (stepS109). The second determination time TBA2 th is about two times thefirst determination time TBA1 th, for example. The second determinationtime TBA2 th corresponds to the increase requisition time when thesecond increasing control is executed. When the sixth determinationtimer T6 is less than the second determination time TBA2 th (step S109:NO), the brake ECU 60 completes the BA processing routine to continuethe second increasing control. When the sixth determination timer T6 isgreater than or equal to the second determination time TBA2 th (stepS109: YES), the brake ECU 60 proceeds to step S106, which is describedabove. That is, the brake ECU 60 terminates the second increasingcontrol and initiates the holding control. Accordingly, in the presentembodiment, the brake ECU 60 also functions as an assist control unitthat performs assist control. Steps 101 to 107 configure an assistingstep.

Next, a BA termination determination processing routine in step S16 willbe described with reference to the flowchart shown in FIG. 14.

In the BA termination determination processing routine, the brake ECU 60determines whether or not the assist control condition satisfaction flagFLG4 is ON (step S120). When the assist control condition satisfactionflag FLG4 is ON (step S120: YES), the assist control is being executed.Thus, the brake ECU 60 increments, by “1”, a seventh determination timerT7 corresponding to the time elapsed from when the execution of theassist control is initiated (step S121). Subsequently, the brake ECU 60determines whether the G sensor value G calculated in step S27 is lessthan the second deceleration determination value G_st, which is set instep S80 (step S122). When the G sensor value G is less than the seconddeceleration determination value G_st (step S122: YES), the brake ECU 60determines that the termination condition of the assist control issatisfied. The brake ECU 60 sets the assist control conditionsatisfaction flag FLG4 and the increase completion flag FLG5 to OFF(step S123). Further, the brake ECU 60 performs decreasing control fordecreasing the braking force applied to the wheels FR, FL, RR, and RL(step S124). Then, the brake ECU 60 terminates the BA terminationdetermination processing routine.

When the G sensor value G is greater than or equal to the seconddeceleration determination value G_st (step S122: NO), the brake ECU 60determines whether or not the increase completion flag FLG5 is ON (stepS125). When the increase completion flag FLG5 is OFF (step S125: NO),the increasing control is being executed. Thus, the brake ECU 60determines whether the following two conditions are satisfied (stepS126).

(Seventh Condition) The seventh determination timer T7 is less than orequal to a termination determination time reference value T7 th.

(Eighth Condition) An ABS flag FLG6 is ON.

The termination determination time reference value T7 th is set to ashorter time than the increase requisition time, which is the executiontime of the increasing control. More specifically, the terminationdetermination time reference value T7 th is set to one half or less ofthe increase requisition time, which is the execution time of theincreasing control. When the first increasing control is executed, thetermination determination time reference value T7 th is set to a value(e.g., 34) corresponding to a time (e.g., 204 milliseconds) that isabout one half the first increase requisition time (e.g., 500milliseconds). When the second increasing control is executed, thetermination determination time reference value T7 th is set to a value(e.g., 68) corresponding to a time (e.g., 408 milliseconds) that isabout one half the second increase requisition time (e.g., 1,000milliseconds).

The ABS flag FLG6 is set to ON when the ABS control is being executed orthe initiating condition of the ABS is satisfied. That is, when the ABScontrol is being executed or the initiating condition of the ABS controlis satisfied, it is determined in step S126 whether the seventhdetermination timer T7 is less than or equal to the terminationdetermination time reference value T7 th.

When at least one of the seventh and eighth conditions is not satisfied(step S126: NO), the brake ECU 60 determines that the terminationcondition of the assist control is not satisfied and completes the BAtermination determination processing routine. When the seventh andeighth conditions are both satisfied (step S126: YES), the brake ECU 60determines that the ABS control has been initiated immediately afterinitiation of the assist control. In this case, the brake ECU 60determines that a sufficiently large braking force can be applied to thewheels FR, FL, RR, and RL just with the driver's depression operationamount of the brake pedal 31. Hence, the brake ECU 60 determines thatthe termination condition of the assist control is satisfied andproceeds to step S123. Accordingly, in the present embodiment, the brakeECU 60 also functions as a termination determination unit thatdetermines, during execution of the assist control, whether thetermination condition of the assist control is satisfied based on atleast one of the vehicle body deceleration (first estimated vehicle bodydeceleration) DV and the G sensor value (second estimated vehicle bodydeceleration) G. Step S126 corresponds to a termination determiningstep.

When the increase completion flag FLG5 is ON (step S125: YES), theholding control is being executed. Thus, the brake ECU 60 acquires abraking force amount (“assisting braking force amount”, hereinafter)which is increased by the execution of the increasing control andapplied to the wheels FR, FL, RR, and RL. Here, the brake ECU 60estimates the assisting braking force amount based on the operation timeand operation speed of the linear solenoid valve 44 and the pump 49. Thebrake ECU 60 adds an increase component value KGba, which corresponds tothe assisting braking force amount, to the second decelerationdetermination value (braking force reference value) G_st, which is setin step S80, and sets the added result as a termination determinationvalue (determination value) KGend (step S127). Further, the brake ECU 60determines whether the following two conditions are satisfied (stepS128).

(Ninth Condition) The G sensor value G is less than a terminationdetermination value KGend.

(Tenth Condition) The ABS flag FLG6 is OFF.

When at least one of the ninth and tenth conditions is not satisfied(step S128: NO), the brake ECU 60 determines that the terminationcondition of the assist control is not satisfied and terminates the BAtermination determination processing routine. When the ninth and tenthconditions are both satisfied (step S128: YES), the brake ECU 60determines that the termination condition of the assist control issatisfied and proceeds to step S123. Accordingly, in the presentembodiment, steps S127 and S128 configure a termination determinationstep.

When the assist control condition satisfaction flag FLG4 is OFF (stepS120: NO), the brake ECU 60 determines that the assist control is notexecuted or the assist control has been terminated. The brake ECU 60resets the seventh determination timer T7 to “0 (zero)” (step S129).Then, the brake ECU 60 terminates the BA termination determinationprocessing routine.

Accordingly, the present embodiment has the advantages described below.

(1) Detection signals from the wheel speed sensors SE2 to SE5 are moreeasily affected by interference (i.e., outer disturbance) between thedrive force transmitted to the wheels FR, FL, RR, and RL and the brakingforce than the detection signal from the vehicle body accelerationsensor SE6. The detection signals from the wheel speed sensors SE2 toSE5 are easily affected by the reaction force (i.e., outer disturbance)received by the wheels FR, FL, RR, and RL from the road surface alongwhich the vehicle is traveling. If the first deceleration determinationvalue DV_st is not corrected, a vehicle body deceleration DV thatincludes a vibration component based on outer disturbance easily exceedsthe first deceleration determination value DV_st as compared with avehicle body deceleration DV that does not include a vibrationcomponent. That is, there is a high probability that assist control willbe initiated in an unintended manner.

Hence, in the present embodiment, when it is determined that a vibrationcomponent based on outer disturbance is included in the vehicle bodydeceleration DV, the first deceleration determination value DV_st is setto a value greater than that when it is determined that a vibrationcomponent based on outer disturbance is not included in the vehicle bodydeceleration DV. Thus, even if a vibration component based on outerdisturbance is included in the vehicle body deceleration DV, the vehiclebody deceleration DV does not easily exceed the first decelerationdetermination value DV_st. Accordingly, when an emergency brakingoperation is not being performed, the unintended initiation of theassist control can be prevented.

(2) When the acquired bad road index Nrw is greater than or equal to“1”, it is determined that the road surface along which the vehicle istraveling is a bad road. In this case, the first decelerationdetermination value DV_st is set to a greater value than that when it isdetermined that the road surface is not a bad road. Thus, when thevehicle travels along a bad road, unintended initiation of the assistcontrol can be prevented.

(3) In the present embodiment, a bad road determination correction valueDVbad, which is used to correct the first deceleration determinationvalue DV_st when the road surface is a bad road, is set to a largervalue as the bad road index Nrw increases. This increases thedetermination accuracy of whether the current depression operation ofthe brake pedal 31 performed by a driver is an emergency brakingoperation.

(4) Even if the bad road index Nrw is “0 (zero)”, a vibration componentbased on a reaction force received by the wheels FR, FL, RR, and RL froma road surface may be included in the vehicle body deceleration DV.Hence, in the present embodiment, when the bad road index Nrw is “0(zero)”, the first deceleration determination value DV_st is set to agreater value than the basic value KDV by the amplitude W_DV of theacquired vehicle body deceleration. Thus, even if a road surface alongwhich the vehicle is traveling is not that uneven and not determined asbeing a bad road, unintended initiation of the assist control can beprevented.

(5) Detection signals from the wheel speed sensors SE2 to SE5 includefine cyclical changes (also referred to as “fluctuation”) regardless ofouter disturbance. Thus, the vehicle body deceleration DV calculatedwith the wheel speed sensors SE2 to SE5 also includes fine cyclicalchanges. Such fine cyclical changes are irrelevant with the influence ofouter disturbance. Thus, there is no need to correct the firstdeceleration determination value DV_st based on fine cyclical changes.Hence, in the present embodiment, the amplitude reference value KW isset as a determination value for determining whether there is aninfluence of outer disturbance. When the amplitude W_DV of the acquiredvehicle body deceleration is less than the amplitude reference value KW,the correction of the first deceleration determination value DV_st basedon the amplitude W_DV is not performed. Therefore, when it is determinedthat there is almost no unevenness in the road surface, the assistcontrol can be initiated at an appropriate timing.

(6) In the present embodiment, when the G sensor value G is greater thanthe sum of the vehicle body deceleration DV and the decelerationspecified value DVth1 or when the vehicle body deceleration DV oncebecomes a negative value, it is determined that the vehicle moved over abump. The first deceleration determination value DV_st is set to a valuegreater than the basic value KDV by the bump determination correctionvalue DVstep (=KDVbad). Therefore, when the vehicle moves over a bump,initiation of unintended assist control can be restricted.

(7) Moreover, the bump determination correction value DVstep whendetermined that the vehicle moved over a bump is the same as the badroad determination correction value DVbad when the bad road index Nrw isdetermined as being “3”. Hence, when the bad road correction flag FLG1and the bump correction flag FLG2 are both set to ON, the firstdeceleration determination value DV_st is corrected based on the bumpdetermination correction value DVstep. That is, the first decelerationdetermination value DV_st is corrected to a greater value. Accordingly,unintended initiation of the assist control can be prevented.

(8) When a signal indicating that the automatic transmission 21 hasundergone downshifting is received from the AT ECU 23, the firstdeceleration determination value DV_st is corrected, during the period(gear shifting specified time) corresponding to the gear shiftingcompletion determination value KTd, to a value that is greater than thatwhen a signal indicating that the automatic transmission 21 hasundergone downshifting is not received. This prevents the downshiftingof the automatic transmission 21 from causing unintended initiation ofthe assist control.

(9) A vehicle has a characteristic in which when the driver depressesthe brake pedal 31 when the vehicle is traveling, the vehicle starts todecelerate and may thereby cause downshifting of the automatictransmission 21. Thus, in the present embodiment, when it is notpossible to communicate with the AT ECU 23, it is determined whether thedriver's depression operation amount of the brake pedal 31 is largeusing the G sensor value G. If the third determination timer T3corresponding to the duration time during which the G sensor value Gremains to be the downshift determination value KGflat exceeds the firsttime T3_1, it is determined that the downshifting of the automatictransmission 21 may be performed. As a result, the first decelerationdetermination value DV_st is corrected to a value greater than that whenit is not determined that there is a possibility that the downshiftingoperation is performed. This prevents the downshifting of the automatictransmission 21 from causing unintended initiation of the assistcontrol.

(10) Immediately after the third determination timer T3 exceeds thefirst time T3_1, the probability of the automatic transmission 21 beingactually downshifted is low, and assist control may actually becomenecessary. Here, if the downshift determination value KGflat is set to amaximum correction value KDVflat1 when the third determination timer T3exceeds the first time T3_1, there is a possibility that the actuallynecessary assist control will not be executed. Hence, in the presentembodiment, when the value of the third determination timer T3 is large,the first deceleration determination value DV_st is set to a value thatis greater than that when the value of the third determination timer T3is small. Thus, immediately after the third determination timer T3exceeds the first time T3_1, the assist control can be properlyinitiated.

(11) In the present embodiment, the brake ECU 60 determines whetherthere is a probability of the automatic transmission 21 performingdownshifting even when communication becomes impossible between the ATECU 23 and the brake ECU 60. If it is determined that the automatictransmission 21 may undergo downshifting, the first decelerationdetermination value DV_st is corrected. Hence, even if a communicationtrouble occurs between the AT ECU 23 and the brake ECU 60, downshiftingof the automatic transmission 21 is prevented from causing unintendedinitiation of the assist control.

(12) The present embodiment acquires a gradient estimated value Gslopeof a road surface along which the vehicle is traveling. The firstdeceleration determination value DV_st is corrected based on thegradient estimated value Gslope. This suppresses variations in theinitiation timing of the correcting control based on the gradient of theroad surface.

(13) When the gradient of a road surface is a positive value, thisindicates that the road surface is a sloped surface directed uphill.When the gradient of a road surface is a negative value, this indicatesthat the road surface is a sloped surface directed downhill. When theroad is a sloped road directed uphill, gravity acting on the vehicleacts as a braking force applied to the vehicle. When the vehicle travelsalong a slope, a deceleration difference corresponding to the gradientof the road surface exists between vehicle body deceleration DV and theG sensor value G. Hence, in the present embodiment, when it isdetermined that the road is a sloped road directed uphill, the firstdeceleration determination value DV_st is set to a value that is greaterthan the basic value KDV, and the second deceleration determinationvalue G_st is set to a value that is less than the basic value KGst.When it is determined that the road is a sloped road directed downhill,the first deceleration determination value DV_st is set to a value thatis less than the basic value KDV, and the second decelerationdetermination value G_st is set to a value that is greater than thebasic value KGst. This suppresses variations in the initiation timing ofassist control caused by the gradient of the road surface.

(14) When a gradient of a road surface is changed and indicates anuphill slope, the braking force resulting from the change in thegradient of the road surface is applied to the front wheels FR and FL,and the wheel speeds VW of the front wheels FR and FL suddenly becomeslow. In contrast, the vehicle body speed VS of the vehicle is notdecelerated as much as the wheel speeds VW of the front wheels FR andFL. Thus, the gradient change DDV of the vehicle body deceleration DVcalculated with the wheel speed sensors SE2 to SE5 is deviated from thegradient change DG of the G sensor value G calculated using the vehiclebody acceleration sensor SE6. Hence, in the present embodiment, when thegradient change DG of the G sensor value is less than the gradientchange reference value KDGlow, it is determined that the gradient of theroad surface has changed and indicates an uphill slope. As a result, thegradient change correction value DVDGlow is set to a value that isgreater than “0 (zero)”. That is, the first deceleration determinationvalue DV_st is corrected to a value greater than that when the gradientchange DG of the G sensor value is greater than or equal to the gradientchange reference value KDGlow, that is, greater than that when it isdetermined that the gradient of the road surface is not changed so as toindicate an uphill slope. This prevents the assist control from beinginitiated in an unintended manner when the gradient of the road surfacechanges thereby indicating an uphill slope.

(15) The gradient change reference value KDGlow is set to a value basedon the difference between the gradient change DDV of vehicle bodydeceleration and the gradient change DG of the G sensor value. Hence, itis possible to increase the accuracy for determining whether or not thegradient of a road surface has changed thereby indicating an uphillslope.

(16) The assist control may be performed in an unintended manner whenthe first deceleration determination value DV_st is corrected based onat least one of the deceleration correction value DVtp, downshiftdetermination correction value DVflat, and gradient change correctionvalue DVDGlow. Hence, in the present embodiment, in the assist controlperformed when the first deceleration determination value DV_st iscorrected based on at least one of the deceleration correction valueDVtp, the downshift determination correction value DVflat, and thegradient change correction value DVDGlow, the increasing speed of thebraking force applied to the wheels FR, FL, RR, and RL becomes slowerthan that for an assist control performed when the decelerationcorrection value DVtp, the downshift determination correction valueDVflat, and the gradient change correction value DVDGlow are “0 (zero)”.Thus, even if assist control is executed when it is determined thatthere is no need to perform the assist control, the driver of thevehicle is less likely to feel uncomfortable due to the execution of theassist control.

(17) During execution of the assist control, the wheel speed sensors SE2to SE5 and the vehicle body acceleration sensor SE6 are used todetermine whether or not the termination condition of the assist controlis satisfied. When the termination condition is satisfied, the assistcontrol is terminated. Accordingly, even if the vehicle does not includea pressure sensor that detects the MC pressure in the master cylinder321, the assist control can be completed at a proper timing.

(18) In the present embodiment, the termination determination valueKGend is obtained by adding the increase component value KGba of thebraking force based on execution of increasing control to the seconddeceleration determination value G_st, which is used for the terminationdetermination of the assist control. During execution of the holdingcontrol, when the G sensor value G that once exceeded the seconddeceleration determination value G_st becomes less than the terminationdetermination value KGend, the driver's depression operation amount ofthe brake pedal 31 is determined as being small, and the assist controlis terminated. Accordingly, the assist control may be terminated whendetermined that the driver intends to decrease the deceleration of thevehicle.

(19) In the present embodiment, when the ABS control is initiated duringexecution of the holding control for the assist control, the holdingcontrol is continued. That is, the assist control is not terminated.Hence, the assist control is not terminated in an inadvertent manneragainst the driver's intentions.

(20) In the present embodiment, if the ABS control is initiated when theseventh determination timer T7 is less than or equal to the terminationdetermination time reference value T7 th during execution of theincreasing control, it is determined that a sufficiently large brakingforce is applied to the wheels FR, FL, RR, and RL by the driver'sdepression operation of the brake pedal 31. Hence, the assist control isterminated. Accordingly, the assist control may be terminated at anappropriate timing.

(21) In the present embodiment, the gradient change DDV of the vehiclebody deceleration when vehicle body deceleration DV becomes greater thanor equal to the first deceleration determination value DV_st is acquiredas a first gradient change DDV1. Then, when the G sensor value G becomesgreater than or equal to the second deceleration reference value G_st,the gradient change DDV of the vehicle body deceleration that iscalculated afterward is acquired as the second gradient change DDV2.When the second gradient change DDV2 is greater than or equal to thefirst gradient change DDV1, it is determined that the depression forceproduced when the driver depresses the brake pedal 31 is high.Therefore, even if the vehicle does not have a pressure sensor thatdetecting an MC pressure in the master cylinder 321, it can bedetermined whether the depression force produced when the driverdepresses the brake pedal 31 is high.

(22) In some vehicles, the change in the G sensor value G based on thedriver's depression operation of the brake pedal 31 is initiated atsubstantially the same time as when the vehicle body deceleration DV isstarted. Hence, in the present embodiment, a second gradient change DDV2is acquired after the fifth determination timer T5, which is renewedafter the first gradient change DDV1 is acquired, exceeds the specifiedwaiting time KT_w. When the second gradient change DDV2 is greater thanor equal to the first gradient change DDV1, it is determined that thedepression force produced when the driver depresses the brake pedal 31is high. Hence, it is possible to increase the accuracy for determiningwhether or not the depression force produced when the driver depressesthe brake pedal 31 is high.

(23) In the present embodiment, even though the initiating condition ofassist control is satisfied, if determined that the depression forceproduced when the driver depresses the brake pedal 31 is high, it isdetermined that assist control is unnecessary, and the assist control isnot started. Accordingly, a case in which the assist control is executedin an unintended manner can be avoided.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIGS. 20 and 21. The second embodiment differs from thefirst embodiment in that the initiation time determination referencevalue KT2 is changed by the load of the vehicle. Accordingly, thedifference will mainly be described hereafter, and like or samereference numerals are given to those components that are the same asthe corresponding components of the first embodiment. Such componentswill not be described.

The load of a vehicle changes in accordance with the number of occupantsin the vehicle and the cargo carried by the vehicle. When the load ofthe vehicle changes, the characteristics of the vehicle also change.Specifically, when the driver depresses the brake pedal 31, the G sensorvalue G, which is calculated with the vehicle body acceleration sensorSE6, changes following, in a slightly delayed manner, the vehicle bodydeceleration DV, which is calculated with the wheel speed sensors SE2 toSE5. However, the timing at which the G sensor value G starts to changedue to the driver's depression operation of the brake pedal 31 isdelayed when the load is heavy from that when the load is light.

Hence, when the initiation time determination reference value KT2 (seestep S89 in FIG. 12), which is used to determined the initiation timingof the assist control, is constant, the G sensor value G exceeds thesecond deceleration determination value G_st after the fifthdetermination timer T5 exceeds the initiation time determinationreference value KT2. In this case, the initiation condition of theassist control is not satisfied. Thus, the assist control is notinitiated.

The braking control processing routine of the present embodimentincludes an initiation time determination reference value settingprocessing that sets the initiation time determination reference valueKT2 to a value corresponding to the load of the vehicle. The initiationtime determination reference value setting processing routine will nowbe described with reference to the flowchart shown in FIG. 20 and themap shown in FIG. 21.

In the initiation time determination reference value setting processingroutine, the brake ECU 60 determines whether the brake switch SW1 is OFF(step S140). When the brake switch SW1 is ON (step S140: NO), the driveris depressing a brake pedal 31. Thus, the brake ECU 60 terminates theinitiation time determination reference value setting processingroutine.

When the brake switch SW1 is OFF (step S140: YES), the driver is notdepressing the brake pedal 31. Thus, the brake ECU 60 acquires the driveforce ET transmitted to the front wheels FR and FL, which are the drivewheels (step S141). For example, the brake ECU 60 acquires the driveforce generated by the engine 12 from the engine ECU 13 and the gearposition of the automatic transmission 21 from the AT ECU 23. Then, thebrake ECU 60 calculates the drive force ET transmitted to the frontwheels FR and FL based on the acquired drive force generated in theengine 12 and the gear position of the automatic transmission 21.Accordingly, in the present embodiment, the brake ECU 60 also functionsas a drive force acquisition unit, which acquires the drive force ETapplied to the front wheels FR and FL, based on the driving operation ofthe engine 12.

Subsequently, the brake ECU 60 estimates a load WW of the vehicle (stepS142). When the load WW of the vehicle is constant, the acceleration ofthe vehicle corresponds to the drive force ET transmitted to the frontwheels FR and FL. In other words, when the drive force ET transmitted tothe front wheels FR and FL is constant, the acceleration of the vehicle,namely, the G sensor value G decreases as the load WW of the vehiclebecomes heavier.

Therefore, the brake ECU 60 acquires the reference value Gbase for the Gsensor value corresponding to the drive force ET acquired in step S141.The reference value Gbase is a theoretical value of the G sensor valueunder the assumption that there is no occupant and cargo in the vehicle.The brake ECU 60 acquires, as an acceleration difference, the difference(=|G−Gbase|) between the G sensor value G calculated in step S28 and thereference value Gbase. Subsequently, the brake ECU 60 acquires the loadWW of the vehicle corresponding to the acquired acceleration differenceusing a third map shown in FIG. 21.

The third map is used to acquire the load WW of the vehiclecorresponding to the acceleration difference. As shown in FIG. 21, thevertical axis of the third map shows the acceleration difference(=|G−Gbase|), and the horizontal axis shows the load WW of the vehicle.When the acceleration difference is less than or equal to a firstdifference ΔG1, the load WW of the vehicle is determined as being “0(zero)”. If the acceleration difference exceeds the first differenceΔG1, an increase in the acceleration difference indicate a larger loadWW of the vehicle. Accordingly, in the present embodiment, the brake ECU60 also functions as a load acquisition unit. Step S142 corresponds to aload acquiring step.

Returning to the flowchart shown in FIG. 20, the brake ECU 60 sets aload correction value HW based on the load WW of the vehicle, which isestimated in step S142 (step S143). As the load WW of the vehiclebecomes heavier, the delay when the G sensor value G starts to changebecomes greater. Thus, in step S143, if the load WW of the vehicle isheavy, the load correction value HW is set to a larger value than whenthe load WW is light using a predetermined arithmetic expression. Here,if the load WW of the vehicle is “0 (zero)”, the load correction valueHW is set to “0 (zero)”.

Subsequently, the brake ECU 60 adds the load correction value HW, whichis set in step S143, to a preset base value KTbase, and the additionresult is set as a initiation time determination reference value KT2(step S144). Accordingly, in the present embodiment, the brake ECU 60also functions as an initiation time setting unit. When the load WW ofthe vehicle is heavy, the initiation time setting unit sets theinitiation time determination reference value KT2 to a larger value thanwhen the load WW is light. Step S144 corresponds to a start time settingstep. Thereafter, the brake ECU 60 completes the initiation timedetermination reference value setting processing routine.

Accordingly, in the present embodiment, the advantages described belowcan be obtained in addition to advantages (1) to (23) of the firstembodiment.

(24) The initiation time determination reference value KT2 is set to avalue corresponding to the load WW of the vehicle. This improves thedetermination accuracy of step S89. Accordingly, when assist control isnecessary, the assist control can be properly initiated.

(25) When the load WW of the vehicle is estimated, the G sensor value Gand the drive force ET transmitted to the front wheels FR and FL, whichare drive wheels, are used when a braking operation is not performed.Thus, there is no need to separately provide a sensor that detects theload WW of the vehicle, and the weight of the vehicle can be estimated.

The above embodiments may be modified to the embodiments describedbelow.

In the above embodiments, the determination processing of step S103 maybe omitted. In this case, the increasing control of the assist controlis set to the first increasing control irrespective of correction of thefirst deceleration determination value DV_st.

A vibration component included in the vehicle body deceleration DV whenthe automatic transmission 21 is downshifted becomes greater as thevehicle body speed VS increases. Hence, the downshift determinationcorrection value DVflat may be increased as the vehicle body speed VSincreases. This lowers the possibility of the assist control beingexecuted in an unintended manner when the automatic transmission 21 isdownshifted as the vehicle travels at a high speed.

In each embodiment, when it is determined that the automatictransmission 21 may be downshifted, the downshift determinationcorrection value DVflat may be set to a preset predetermined valueirrespective of the value of the third determination timer T3.

In each embodiment, the processing operations of steps S66 to S69 may beomitted. This can also correct the first deceleration determinationvalue DV_st by receiving a downshift signal from the AT ECU 23.

In each embodiment, if the vehicle cannot receive information related tothe downshifting of the transmission like when the transmission of thevehicle is a manual transmission, the processing operations of steps S57to S65 may be omitted. In this case, the determination processing instep S66 is executed after the processing operations of step S52 andstep S56.

In each embodiment, when the transmission of the vehicle is acontinuously variable automatic transmission, a vibration componentbased on outer disturbance caused by a downshifting operation of theautomatic transmission included in the vehicle body deceleration DV issubtle. Hence, the processing operations in steps S57 to S69 may beomitted.

In each embodiment, the first deceleration determination value DV_stdoes not have to be corrected based on the bump determination correctionvalue DVstep.

In each embodiment, the bump determination correction value DVstep doesnot have to be set to “0 (zero)” in step S51. In this case, the bumpcorrection flag FLG2 is also set to OFF in step S52. Thus, correction ofthe first deceleration determination value DV_st based on the bumpdetermination correction value DVstep is not performed.

In each embodiment, the bump determination correction value DVstep maybe set to a value greater than the third correction value KDVbad3 instep S55.

Further, the bump determination correction value DVstep may be set to avalue less than the third correction value KDVbad3. In this case,however, if the bad road correction flag FLG1 is ON, the firstdeceleration determination value DV_st may be corrected based on the badroad determination correction value DVbad.

In each embodiment, the second time determination value T2 th may bedecreased as the vehicle body speed VS increases. This is because anincrease in the vehicle body speed VS shortens the time required for thevehicle to move past a bump. An estimated time from when the frontwheels FR and FL of the vehicle move over a bump to when the rear wheelsRR and RL pass by the bump may be calculated based on the vehicle bodyspeed VS and the wheel base length of the vehicle, and the estimatedtime may be set as the second time determination value T2 th.

In each embodiment, the determination processing in step S46 may beomitted. If the first deceleration determination value DV_st iscorrected based on the amplitude W_DV when the downshift determinationcorrection value DVflat and the gradient change correction value DVDGloware “0 (zero)”, the first increasing control may be performed when theinitiating condition of the assist control is satisfied.

In each embodiment, the amplitude W_DV of the vehicle body decelerationDV when the bad road index Nrw is “0 (zero)” does not have to becalculated. That is, the correction of the first decelerationdetermination value DV_st based on the amplitude W_DV does not have tobe performed.

In each embodiment, step S31 may be omitted. Even in this case, the badroad correction flag FLG1 is set to OFF in step S32. Thus, the firstdeceleration determination value DV_st is not corrected based on the badroad determination correction value DVbad.

In each embodiment, the first deceleration determination value DV_stdoes not have to be corrected based on the bad road index Nrw. In thiscase, the first deceleration determination value DV_st can be correctedby the amplitude W_DV of the vehicle body deceleration DV.

In each embodiment, a vertical acceleration sensor for detecting thevertical acceleration of the vehicle may be provided in the vehicle.Further, the bad road index Nrw of a road surface may be calculatedbased on changes in the vertical acceleration, which is based on thedetection signal from the vertical acceleration sensor.

In each embodiment, the gradient change reference value KDGlow may be apredetermined value that is preset through an experiment or asimulation. In this case, the second map shown in FIG. 7 is notnecessary.

In each embodiment, the first deceleration determination value DV_st maynot be corrected based on the gradient change correction value DVDGlow.

In each embodiment, the gradient estimated value Gslope may be set basedon the difference between the vehicle body deceleration DV and the Gsensor value G when a driver starts depressing the brake pedal 31. Inthis case, the first deceleration determination value DV_st and thesecond deceleration determination value G_st may be readily corrected inaccordance with the gradient of the road surface.

However, in this correction method, the correction accuracy is low ascompared with each of the above embodiments. Hence, before the gradientestimated value Gslope is acquired by the method of any of the aboveembodiments, the first deceleration determination value DV_st and thesecond deceleration determination value G_st are corrected based on thedifference between the vehicle body deceleration DV and the G sensorvalue G when the depression operation of the brake pedal 31 is started.After the gradient estimated value Gslope is acquired by the method ofany of the above embodiments, the first deceleration determination valueDV_st and the second deceleration determination value G_st may becorrected based on the gradient estimated value Gslope.

In each embodiment, when gradients of road surfaces along which thevehicle travels are stored in a navigation device (not shown) of thevehicle, the gradient of the road surface may be acquired from thenavigation device. Further, the first deceleration determination valueDV_st and the second deceleration determination value G_st may becorrected based on the gradient.

In each embodiment, the second deceleration determination value G_stdoes not have to be corrected based on the gradient estimated valueGslope.

In each embodiment, in step S126, instead of determining whether or notthe ABS flag FLG6 is ON, it may be determined whether or not the vehiclebody deceleration DV is greater than or equal to the decelerationcorresponding to the road surface limit (e.g., 1.2 G).

In each embodiment, in step S126, it may be determined whether or notthe ABS control has been initiated during execution of increasingcontrol. In this case, the termination determination time referencevalue T7 th becomes a value corresponding to the first increaserequisition time (or second increase requisition time).

In each embodiment, the determination processing of step S126 may beomitted.

In each embodiment, in step S128, it may be determined only whether ornot the G sensor value G is less than the termination determinationvalue KGend. In this case, if the G sensor value G is less than thetermination determination value KGend, the assist control is terminatedirrespective of whether ABS control is executed.

In each embodiment, the termination determination value KGend is a sumof the second deceleration determination value G_st and the increasecomponent value KGba. That is, the second deceleration determinationvalue G_st corresponds to a braking force reference value. However, thebraking force reference value may differ from the second decelerationdetermination value G_st.

In each embodiment, the determination processing in step S122 may beomitted. In this case, the determination processing of step S125 isperformed after the processing of step S121 is executed.

In each embodiment, among the two conditions of step S89, the sixthcondition may be “the fifth determination timer T5 is less than or equalto the initiation time determination reference value KT2”. In thismanner, the specified waiting time KT_w is not necessary.

In each embodiment, when the initiating condition for the assist controlis satisfied, the assist control may be performed irrespective of themagnitude of the depression force produced when the driver depresses thebrake pedal 31. In this case, the processing operations in steps S86 andS90 may be omitted.

Some vehicles are provided with detection sensors that detect the numberof occupants in the vehicle. In such a vehicle, the number of occupantsmay be acquired based on the detection signal from the detection sensor,and the load WW may be estimated based on the number of occupants.

In the second embodiment, the load WW of the vehicle is estimated byreading the state when the vehicle is traveling. However, the presentinvention is not limited in such a manner, and the load WW may beacquired by reading load data that is input when the vehicle ismanufactured or load data that is input after the vehicle ismanufactured.

In each embodiment, the vehicle may be a rear wheel drive vehicle, inwhich the rear wheels RR and RL are the drive wheels, or a four wheeldrive vehicle, in which the wheels FR, FL, RR, and RL are all drivewheels.

In each embodiment, the power source of the vehicle may be a motor.

The present invention may be embodied in a braking control device for avehicle provided with a pressure sensor that detects the MC pressure inthe master cylinder 321. The braking control processing shown in FIG. 8may be executed when a failure occurs in the pressure sensor.

1. A braking control device for a vehicle comprising: a firstdeceleration calculation unit that calculates a first estimated vehiclebody deceleration by using a detection signal of a wheel speed sensorarranged on the vehicle; a second deceleration calculation unit thatcalculates a second estimated vehicle body deceleration by using adetection signal of a vehicle body acceleration sensor arranged on thevehicle; an assist control unit that initiates an assist control, whichassists increasing of a braking force applied to a wheel of the vehicleif a brake pedal of the vehicle is operated, when the first estimatedvehicle body deceleration exceeds a first deceleration determinationvalue and the second estimated vehicle body deceleration exceeds asecond deceleration determination value during operation of the brakepedal of the vehicle; and a termination determination unit thatdetermines whether or not a termination condition of the assist controlis satisfied based on at least one of the first estimated vehicle bodydeceleration and the second estimated vehicle body deceleration duringexecution of the assist control, wherein the assist control unitterminates the assist control when the termination determination unitdetermines that the termination condition is satisfied.
 2. The brakingcontrol device for a vehicle according to claim 1, wherein the assistcontrol includes an increasing control, which increases the brakingforce applied to the wheel, and a holding control, which holds thebraking force applied to the wheel after the increasing control isexecuted, the termination determination unit acquires a determinationvalue by adding an increase component of the braking force, which isbased on the execution of the increasing control, to a braking forcereference value set as a reference for determining, when the assistcontrol has not yet been executed, whether or not an operation amount ofthe brake pedal has decreased, and the termination determination unitdetermines, during execution of the holding control, that thetermination condition of the assist control has been satisfied when thesecond estimated vehicle body deceleration calculated by the seconddeceleration calculation unit is less than the determination value. 3.The braking control device for a vehicle according to claim 2, furthercomprising an ABS control unit that performs anti-lock braking control,which restricts locking of the wheel, wherein the terminationdetermination unit prohibits, during execution of the holding control,the termination condition of the assist control from being satisfiedwhen the ABS control unit is executing the anti-lock braking control. 4.The braking control device for a vehicle according to claim 1, whereinthe assist control includes an increasing control, which increases thebraking force applied to the wheel, and a holding control, which holdsthe braking force applied to the wheel after the increasing control isexecuted, wherein the increasing control is executed during a presetincrease requisition time, the braking control device further comprisesan ABS control unit that performs anti-lock braking control, whichrestricts locking of the wheel, wherein when an elapsed time from whenthe increasing control is initiated is shorter than a terminationdetermination time reference value, which is set to be shorter than theincrease requisition time, the termination determination unit determinesthat the termination condition of the assist control is satisfied uponsatisfaction of any one of the conditions of the ABS control unitstarting the anti-lock braking control, and the first estimated vehiclebody deceleration calculated by the first deceleration calculation unitis greater than or equal to a deceleration that corresponds to a roadsurface limit.
 5. A braking control method for a vehicle comprising: afirst deceleration calculating step of calculating a first estimatedvehicle body deceleration by using a detection signal of a wheel speedsensor arranged on the vehicle; a second deceleration calculating stepof calculating a second estimated vehicle body deceleration of thevehicle by using a detection signal of a vehicle body accelerationsensor provided on the vehicle; an assisting step of initiating anassist control, which assists increasing of a braking force applied to awheel of the vehicle if a brake pedal of the vehicle is operated, whenthe first estimated vehicle body deceleration exceeds a firstdeceleration determination value and the second estimated vehicle bodydeceleration exceeds a second deceleration determination value duringoperation of the brake pedal of the vehicle; and a terminationdetermination step of determining whether or not a termination conditionof the assist control is satisfied based on at least one of the firstestimated vehicle body deceleration and the second estimated vehiclebody deceleration, wherein the assisting step terminates the assistcontrol when determined in the termination determination step that thetermination condition is satisfied.